If you end up in a situation where you want to grab an old commit (from some other branch, even) but don’t know the commit hash, you want to access the reflog. It allows you to access old commits easily:

git reflog | head -200 | grep TMP

Will show info on all the commits within the last 200 that has a message containing TMP. This is really useful if you’re using some temporary hack that you want to apply/remove repeatedly without keeping it in a branch.