I agree about one meal being a lapse, 2 a restart.
It also depends whether you've had carbs. That makes it more of a restart in my book. Also, I'd say if the lapses are happening regularly, it's time for a restart because it basically means you're not on the plan. Though I'm sure you could still lose weight that way.
Anything consumed accidentally doesn't count at all to me - i.e. if someone gives you the wrong drink.