Today my git mergetool stopped working. When I ran git mergetool I was greeted with:

git config option merge.tool set to unknown tool: --global
Resetting to default...

This message is displayed because 'merge.tool' is not configured.
See 'git mergetool --tool-help' or 'git help config' for more details.
'git mergetool' will now attempt to use one of the following tools:
tortoisemerge emerge vimdiff
No known merge tool is available.

Somehow (seriously, I have no idea how I did this) I had created a setting in one of my repositories set my mergetool to --global.

If you happen to get yourself into the same issue, run git config --list to see what your config settings are.

For me, I had a rogue merge.tool entry, so I just had to run git config --unset merge.tool which deleted the entry and I was off again.