Under certain circumstances, stale sync events can occur, and those events will then accumulate in the event queue list (see attached screenshot). These can occur in many

different circumstances. Here is one case where this was detected, using eZ Publish 4.7 / eZ Content Staging 4.7.1:

1. Source & target: installed fix from issue EZP-24066;
2. Source & target: modified content.ini.append.php:

[VersionManagement]
DefaultVersionHistoryLimit=2

3. Source: created a new article, with 4 versions;
4. Source: Synced the resulting 4 events. Got he following output:

Event synchronisation action results :

Object 100 to be synchronised to feed 47ezfc_cs2.com: failure (HTTP error 400 (Bad Request)) [Event 1]
Object 100 to be synchronised to feed 47ezfc_cs2.com: failure (Can not serialize object to be sent) [Event 2]
Object 100 succesfully synchronised to feed 47ezfc_cs2.com [Event 3]
Object 100 succesfully synchronised to feed 47ezfc_cs2.com [Event 4]

Events 3 and 4 are correctly processed, but events 1 and 2 are invalid and will remain on the queue.