In some cases, reverting a file (undo checkout) will result in ClearCase stop recognizing the file as a version controlled file.
This can happen when *all* the conditions below are met:
* the file is in a snapshot view
* the checked out file is the first revision on a branch
* ah-clearcase-rmbranch-on-revert-flag is set to t (so vc-clearcase.el will try to remove the branch when reverting).
A workaround is to run an update in the directory, this will make ClearCase recognize the file again.