It worked for these 10 cases. It didn't work for the 11th case. It was probably never right in the first place.

That one is counterintuitive, but true surprisingly often. Maybe not most of the time, but more often than you might think. And it picks the worst times to be right, let me tell you. Especially if it reveals a mistake in the math underlying everything you've been doing....

The solution, I suppose, is to learn to enjoy rewriting.