What's your threshold for "bad?"

My own view is that if the service is actually *bad*, I ask to speak with the manager, and tell them why I'm not tipping.

If I'm *sure* never coming back there again (because of bad service AND bad food), I might tip a nominal small amount (to make it clear that I wasn't just forgetting to tip) and not leave a tip, but it's never actually happened to me. Decent enough service to leave the usual 15% despite bad enouh food I'd never come back is sadly not that uncommon -- but it's unreasonable to penalize the server for the bad food. Having to talk to the manager about an especially bad server has also happened, but only a few times.