There's more to it than just knowing it's broken. I want to know why and how it happened. Of stuff needs to be tested after a merge; that's how we discovered there was something wrong in the first place. But I don't want to redo the work that I already did; I want the merge to be correct. In this case, it seems the merge did something it should never have done, and it was not clear why this happened.

Still, the commits that were merged both still existed, to it was easy to redo the merge. I shudder to think what would have happened if this had been a rebase; then the original commit might have been gone.