When experimenting with #4473, I requested two merges in parallel.. neither of those worked. Then @pingou restarted the workers, one pull request got merged - and the other one not, while the branch was still deleted from my fork (I've checked the "Delete branch after merging").

The error UI message after following merge attempt:
Branch api-dont-list-group-projects could not be found in the repo forks/praiskup/copr/copr

So I rebased locally, pushed again (without --force, and successfully) and pagure allowed me to merge again. I think that the branch should ever be deleted if and only if the data were merged (the status quo is not secure because I could easily have my local copy of that branch lost).

The affected pull-request (now merged, though):
https://pagure.io/copr/copr/pull-request/792