When repository caching is enabled and the branch to be merged is not in the repository cache, the merging fails with the error:

2013-08-26 13:02:19,473 ERROR [3-DelayedChangeDetectionThread:pool-6-thread-28] [ChainExecutionManagerImpl] Failed to merge for plan-result: TESTA-TSTA-24. Merge command error: fatal: 14437b95ee1aaaebd7569f1efa4000dd22ee7e28 - not something we can merge

Going through at the build logs, the issue is caused by the fact that Bamboo fetches only the current branch and as such it can not be merged.

'C:\Program Files (x86)\Git\cmd\git.exe' fetch D:\repositoryclients\git\webapplication +refs/heads/firstbranch:refs/heads/firstbranch --update-head-ok --progress --verbose

With repository caching disabled, the firstbranch will be replaced with an asteriks which fetches all the branches