I can reproduce this consistently at least on Firefox:

1. Open a message in action=edit, e.g. a MediaWiki: /en page, on translatewiki.net
2. Click to get the popup to edit the documentation
3. Save the change

The whole page is closed and the window's URL changes to https://translatewiki.net/w/api.php , with content like

{"edit":{"result":"Success","pageid":3171772,"title":"MediaWiki:Wikibase-statementview-referencesheading-pendingcountersubject/qqq","contentmodel":"wikitext","oldrevid":4710766,"newrevid":rELUA476339174061,"newtimestamp":"2013-06-07T19:57:36Z"}}

The edit is saved.

Version: unspecified
Severity: normal
URL: https://translatewiki.net/wiki/Special:Random/MediaWiki
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=52541