I just learned git bisect from https://ohmygit.org/! You run it, then checkout other commits all over the project, and mark them with git bisect good or git bisect bad. Then it paints all commits that led to the good one as good, and all the ones after the bad one as bad, so you just keep narrowing your window by playing checkout Jezzball until there’s only one commit left: the one that introduced the bad state.