Since D9912 Diffusion no longer works, because we once merged several repositories together. In all the complexity that happened some branches were duplicated.
{F177066}
{F177068}
The actual error:
{F177070}
Adding a setLimit(1) did the trick, but I'd rather have a concrete upstream fix. Also, I think it's quite hard to reproduce.
(oh, it's a Mercurial repo, by the way)