Whenever the reply tool fails to open on an old revision for any reason, it keeps sending "action=discussiontoolscompare" API requests forever. This should only occur while the reply tool is open (to notify the user about new comments in the same section).
For example, I can reproduce on https://en.wikipedia.org/w/index.php?title=Wikipedia:Wikipedia_Signpost/2022-04-24/In_the_media&oldid=1089911778 (the reply tool fails there due to T259824).
Additionally, in some cases the following error occurs for every API request: (Originally reported at T312828#8076979 by @Novem_Linguae)
jQuery.Deferred exception: Cannot read properties of null (reading 'then') TypeError: Cannot read properties of null (reading 'then')
at CommentController.js.CommentController.updateNewCommentsWarning (https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core%7Cjquery.ui&skin=vector&version=1n2jy:48:439)
at https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core%7Cjquery.ui&skin=vector&version=1n2jy:43:362
at mightThrow (https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core%7Cjquery.ui&skin=vector&version=1n2jy:338:881)
at process (https://en.wikipedia.org/w/load.php?lang=en&modules=ext.discussionTools.init%7Cjquery%2Coojs-ui-core%7Cjquery.ui&skin=vector&version=1n2jy:339:544)Curiously it only occurs in some cases, e.g. it happens for the last comment at https://en.wikipedia.org/w/index.php?title=Wikipedia_talk:New_pages_patrol/Coordination&oldid=1098068027 (failing due to T313093), but not in the other example.