Spotted at random. On a closed wiki, only a couple of instances (then a few more of me trying it myself).
Urls were all of the form na.wikibooks.org/wiki/Special:Contributions/<ip>. And only a singular IP at that, others didn't seem to trigger it. Can't find any entries in the block log for said IP, nor is it in the user table (I was thinking maybe name/IP clash?), or recentchanges.
#0 /srv/mediawiki/php-1.31.0-wmf.23/includes/Block.php(462): User::newFromAnyId(string, string, NULL)
#1 /srv/mediawiki/php-1.31.0-wmf.23/includes/Block.php(492): Block->initFromRow(stdClass)
#2 /srv/mediawiki/php-1.31.0-wmf.23/includes/Block.php(362): Block::newFromRow(stdClass)
#3 /srv/mediawiki/php-1.31.0-wmf.23/includes/Block.php(1195): Block->newLoad(User)
#4 /srv/mediawiki/php-1.31.0-wmf.23/includes/specials/SpecialContributions.php(309): Block::newFromTarget(User, User)
#5 /srv/mediawiki/php-1.31.0-wmf.23/includes/specials/SpecialContributions.php(100): SpecialContributions->contributionsSub(User)
#6 /srv/mediawiki/php-1.31.0-wmf.23/includes/specialpage/SpecialPage.php(522): SpecialContributions->execute(string)
#7 /srv/mediawiki/php-1.31.0-wmf.23/includes/specialpage/SpecialPageFactory.php(579): SpecialPage->run(string)
#8 /srv/mediawiki/php-1.31.0-wmf.23/includes/MediaWiki.php(288): SpecialPageFactory::executePath(Title, RequestContext)
#9 /srv/mediawiki/php-1.31.0-wmf.23/includes/MediaWiki.php(861): MediaWiki->performRequest()
#10 /srv/mediawiki/php-1.31.0-wmf.23/includes/MediaWiki.php(524): MediaWiki->main()
#11 /srv/mediawiki/php-1.31.0-wmf.23/index.php(42): MediaWiki->run()
#12 /srv/mediawiki/w/index.php(3): include(string)
#13 {main}