screenshot 1
In my watchlist, I clicked the "diff" link for an edited summary (screenshot 1), url:
https://www.mediawiki.org/w/index.php?title=Talk:Sandbox&action=compare-postsummary-revisions&workflow=rvgf6obpjzqal6xl&topicsummary_newRevision=rvhjlwe3g91164db
and received an error page (screenshot 2) with text:
[bcce47ea] 2014-05-27 16:23:42: Fatal exception of type Flow\Exception\FlowException
Version: unspecified
Severity: normal
Attached: