Scenario:

a plan configured with gatekeeper auto-merging.

Symptom:

after successfully building the feature branch, the push back to master fails with the error message "Exception during push: Can't guess branch name for revision <some_revision> when trying to perform push", where <some_revision> is a Git commit hash. Doing a git branch --contains shows that revision to only be on the gatekeeper branch, so the error message given by Bamboo doesn't make sense.

Example: https://jira-bamboo.internal.atlassian.com/browse/MASTERONE-CIFUNCHAL42-101 (screenshot attached in case that build has been purged).

Workaround

Go to Bamboo overview / Repository Settings / Manage caches
You should be able to locate the revision sha id that causes the "Can't guess branch name" and it should probably be in "Unused" status. Delete it and try to rebuild your plan, it should work!