Deleting two branches in a row can cause buildbot to hang in indeterminate state...
Deleting two branches back to back can sometimes cause buildbot to not register the deletion of the second branch. It continues to poll for this branch for the gitpoller to throw an error when it doesn't exist. This cause no new branches to be built.
I believe the solution is to on the buildbot machine get list of current branches and make sure the deleted on is not present. If it is still present sleep for a second then poll again.
(from redmine: created on 2013-01-30, closed on 2015-09-14)