I was performing a rebase while KDevelop was open with one of the source files opened and I kept getting this abhorrent message:

(daniel@loudmouth)$ git rebase -i master
fatal: Unable to create '/home/daniel/proj/sys/gcc/git/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.

I would check, but by the type I typed ll /home/daniel/proj/sys/gcc/git/.git/index.lock, the file no longer existed.  I closed KDevelop and repeated the command with success.

I realize that KDevelop detects alteration of files, but it's response should not be to break the very normal process that is altering them.