A doctor who lapses and injures her patient, and a driver who lapses and causes an accident, are liable under negligence law for the harm done. But lapse is not necessarily negligence, since reasonable people lapse from time to time. We show that tort liability for "reasonable" lapses distorts doctors’, drivers’, and manufacturers’ incentives to take care. Furthermore, such liability provides potential injurers with incentives to substitute activities which are less prone to lapses with activities which are more prone to lapses, even if such substitution is inefficient. We propose several solutions to the inefficiencies that result from liability for lapses, and argue that under certain circumstances injurers should be entitled to invoke a "lapse defense."