Set task manager sorting to manual and drag and drop windows that have a counter notification on them. The counter is then added to other windows. See attachment for a screencast.
Created attachment 102881 [details] Screencast
Kai, some state updating problem with the badge code?
Might have been the same root-cause as when windows move between tasks (ie. delegate re-creation with a badge already set). This should be fixed in 5.8.4, feel free to re-open if the issue persists with this version, I cannot reproduce it here.