When you interrupt a commit to a smart server using ctrl-C, occasionally it will leave the branch locked. This is difficult to reproduce as it seems to depend on exactly when you hit ctrl-C, but I can get it to happen roughtly 1 in 10 times. See the attached log for the traceback you get when it happens, plus the subsequent error about the lock.
I've reproduced this with Bazaar 2.0.1 and 2.1.0b3 using Python 2.5 on Windows XP SP3, as well as with 2.0.3 on OS X.
To reproduce:
bzr init-repo --no-trees testrepo
bzr init testrepo/branch1
bzr serve --allow-writes &
bzr checkout --lightweight bzr://localhost/testrepo/branch1
cd branch1
Now repeat the following and eventually you'll get an error "Could not acquire lock":
bzr commit -m "test" --unchanged
Hit ctrl-C sometime before the "Committing to:" message appears.
(Also, the error message is wrong as it does not say how to break the lock. However, that's already covered by bug 250451.)
The wrong break lock message is bug 250451.