The query

https://en.wikipedia.org/w/api.php?action=query&list=watchlist&wlprop=loginfo&wllimit=max

shows the following output for me:

<?xml version="1.0"?>
<api servedby="srv265">

<error code="internal_api_error_MWException" info="Exception Caught: Internal error in ApiResult::setIndexedTagName: Bad parameter" xml:space="preserve">

#0 /usr/local/apache/common-local/php-1.19/includes/api/ApiBase.php(1300): wfDebugDieBacktrace('Internal error ...')
#1 /usr/local/apache/common-local/php-1.19/includes/api/ApiResult.php(197): ApiBase::dieDebug('ApiResult::setI...', 'Bad parameter')
#2 /usr/local/apache/common-local/php-1.19/includes/api/ApiQueryLogEvents.php(264): ApiResult->setIndexedTagName('???[edit=autoco...', 'param')
#3 /usr/local/apache/common-local/php-1.19/includes/api/ApiQueryWatchlist.php(306): ApiQueryLogEvents::addLogParams(Object(ApiResult), Array, '???[edit=autoco...', 'protect', 'protect', '20120403012329')
#4 /usr/local/apache/common-local/php-1.19/includes/api/ApiQueryWatchlist.php(201): ApiQueryWatchlist->extractRowInfo(Object(stdClass))
#5 /usr/local/apache/common-local/php-1.19/includes/api/ApiQueryWatchlist.php(40): ApiQueryWatchlist->run()
#6 /usr/local/apache/common-local/php-1.19/includes/api/ApiQuery.php(266): ApiQueryWatchlist->execute()
#7 /usr/local/apache/common-local/php-1.19/includes/api/ApiMain.php(705): ApiQuery->execute()
#8 /usr/local/apache/common-local/php-1.19/includes/api/ApiMain.php(360): ApiMain->executeAction()
#9 /usr/local/apache/common-local/php-1.19/includes/api/ApiMain.php(344): ApiMain->executeActionWithErrorHandling()
#10 /usr/local/apache/common-local/php-1.19/api.php(117): ApiMain->execute()
#11 /usr/local/apache/common-local/live-1.5/api.php(3): require('/usr/local/apac...')
#12 {main}

</error>
</api>

It seems to happen when a watchlist entry refers to a protection, but not when it's an edit or a move.

I can replicate the error on my local wiki (although without the stack trace) that runs from master. And showing the same protection using list=recentchanges works fine.

Version: unspecified
Severity: normal