Probably the most popular error in TortoiseGit is:
git did not exit cleanly (exit code 128)
And all, it tells you, is that something was wrong when calling
git.exe internally. That's pretty not much, don't you think? Well, this StackOverflow question may help you, at least in certain situations. It has 14 answers (from dumbest, like not updating program to newest version) to quite interesting one. This could be a good starting point, before looking somewhere else.