Hey, I'm arguing in favour of contracts and assertions here... XD

I even considered formal verification, if under another name.

Ed.

I think I hit a posting limit. After reading dataflow's answer to gp I think I interpreted ggp differently from both of you.

I meant quite literally neither assertions nor unit tests prevent you from breaking things. Of course they'll help you to catch those mistakes before release. I didn't think that counts as prevention.