Although bzr displays a stack trace, it does appear that the branch *is* successfully pushed (since "bzr diff --old lp:~jamesodhunt/upstart/ignore-me-testing321", for example, reports no output), however I would like confirmation that this bug isn't fatal to the pushed branch!
@jelmer: you are correct - the problem is caused by the verbose option:
# SUCCESS
$ bzr push lp:~jamesodhunt/upstart/ignore-me-testing123
# FAILS
$ bzr push -v lp:~jamesodhunt/upstart/ignore-me-testing321
Although bzr displays a stack trace, it does appear that the branch *is* successfully pushed (since "bzr diff --old lp:~jamesodhunt/upstart/ignore-me-testing321", for example, reports no output), however I would like confirmation that this bug isn't fatal to the pushed branch!