I was trying to sync a Perforce client and got this error:
$ p4 sync Can't clobber writable file: /home/joe/p4_workspace/foobar.txt
Perforce keeps its files read-only until they are opened for editing. I had for some reason made this file writeable and Perforce was complaining about that with this error message.
The error was fixed and the sync worked once I made the file read-only:
$ chmod -w /home/joe/p4_workspace/foobar.txt