Found this in the logs. It is not reproducible by visiting the url - either because it was one-time glitch or the diff is now cached. Using HHVM.
```
2015-10-22 07:11:47 translatewiki.net translatewiki_net-bw_: [f1f535ed] /w/i.php?title=Template:Desc/doc&diff=5608611&oldid=4328897 ErrorException from line 444 of /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php: PHP Notice: Undefined property: DiffEngine::$seq
#0 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(444): MWExceptionHandler::handleError(integer, string, string, integer, array, array)
#1 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(544): DiffEngine->diag(integer, integer, integer, integer, integer)
#2 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(344): DiffEngine->compareSeq(integer, integer, integer, integer)
#3 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(227): DiffEngine->diffLocal(array, array)
#4 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(721): DiffEngine->diff(array, array)
#5 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(859): Diff->__construct(array, array)
#6 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php(981): MappedDiff->__construct(array, array, array, array)
#7 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/TableDiffFormatter.php(200): WordLevelDiff->__construct(array, array)
#8 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DiffFormatter.php(140): TableDiffFormatter->changed(array, array)
#9 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DiffFormatter.php(84): DiffFormatter->block(integer, integer, integer, integer, array)
#10 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DifferenceEngine.php(880): DiffFormatter->format(Diff)
#11 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DifferenceEngine.php(796): DifferenceEngine->generateTextDiffBody(string, string)
#12 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DifferenceEngine.php(727): DifferenceEngine->generateContentDiffBody(WikitextContent, WikitextContent)
#13 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DifferenceEngine.php(661): DifferenceEngine->getDiffBody()
#14 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DifferenceEngine.php(631): DifferenceEngine->getDiff(string, string, string)
#15 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DifferenceEngine.php(453): DifferenceEngine->showDiff(string, string, string)
#16 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/page/Article.php(797): DifferenceEngine->showDiffPage(boolean)
#17 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/page/Article.php(508): Article->showDiffPage()
#18 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/actions/ViewAction.php(44): Article->view()
#19 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/MediaWiki.php(457): ViewAction->show()
#20 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/MediaWiki.php(254): MediaWiki->performAction(Article, Title)
#21 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/MediaWiki.php(668): MediaWiki->performRequest()
#22 /srv/mediawiki/tags/2015-10-18_19:48:39/includes/MediaWiki.php(474): MediaWiki->main()
#23 /srv/mediawiki/tags/2015-10-18_19:48:39/index.php(41): MediaWiki->run()
#24 {main}
```
```
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000001] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 444
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000002] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 448
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000003] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000004] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000005] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 479
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000006] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 444
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000007] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 479
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000008] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 489
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000009] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 498
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000010] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000011] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000012] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000013] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 479
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000014] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000015] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000016] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000017] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000018] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 479
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000019] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000020] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000021] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000022] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000023] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000024] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 479
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000025] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000026] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000027] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000028] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000029] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000030] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000031] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 479
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000032] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000033] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000034] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000035] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000036] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000037] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000038] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000039] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000040] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000041] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000042] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000043] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000044] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000045] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000046] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000047] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
[Thu Oct 22 09:11:47 2015] [hphp] [10318:7f3657fff700:1:000048] [] \nNotice: Undefined property: DiffEngine::$seq in /srv/mediawiki/tags/2015-10-18_19:48:39/includes/diff/DairikiDiff.php on line 418
```