From the NeverIsNeverNever page on the C2 wiki:
I know that if you agree NEVER to let the unit tests drop below 100%, you'll only do it that one time when you just couldn't figure out an incremental way to change all the deductions from negative to positive. If you agree NEVER to keep a task open for two weeks, you have a better chance of finding the way to do it incrementally, and that one time you'll make sure you've sucked every idea out of the universe before giving in.
When I say something should never be done, it should mean that you'll never do it unless you really have to.
And if you really have to, you'll ask everyone you know first so you still won't have to.
And if you still have to, you'll be looking over your shoulder scared to death that I'm going to materialize there and say "why didn't you just ..." and be RIGHT, and EVERYONE will know you're an idiot.
When you're sure that you'll be able to say "because X" and I'll quietly lower my eyes and say "oh" and de-rez back to wherever I come from ... then break the rule.
Then do one more thing. When it's over, and you've suffered - as you will - for breaking the rule, think back and figure out what you could have done, and learn the deeper meaning of NEVER.