CVS can use sticky tags, which has similar functionality to a branch in that it creates a separate working copy to each revision. It can cause an error:

cvs commit: sticky tag 'BG' for file xxx is not a branch

This happens when attempting to check in data against a tag - which is not possible.

Remove a sticky tag

Backup any modified files that cannot be checked in due to the error above and update the repo to the latest version of trunk:

$ cd path/to/repo
$ cvs update -A