I'm nominating this for 2.5.1 because it's a regression from the darcs-
2.4.x line (perhaps it'd be more realistic to bump it).

The symptom seems to kick in when you have at least one tag in the repo 
you're applying to (don't know if it has to be in the shared context or 
not), but you're missing the tag the bundle is against.

I have a regression test which I'll submit shortly

We need a fix for this soon if it's to make 2.5.1. (Also, don't forget the 
promised test.)

Sorry, just saw the test is in patch509.

Reopening this issue. We have recently seen situations where this
re-appeared that aren't covered by the tests. The symptom is either
darcs hanging indefinitely, or crashing with the dreaded "failed to
commute common patches" error. A preliminary fix is contained in
patch1901 but it may need cleanup and doesn't yet cover context
files, which have the same problem.

The mentioned patch that fixes this is accepted together with a few 
follow-ups that also fix context files.