Yes, everyone has his/her bad moments, bad days, it depends

on how bad it was. If the waiter/waitress' service was slow because he/she was too busy, or she/he dids not have a smile on the face at all, or the food was bad(not their fault of course) I can understand that and still tip 20%, but if he/she gave me attitude with no reason or treated me without respect, or being rude and way out of line (such in he2 MM's case), that might put me in my "bad moment" and won't tip well. Please understand, just like the servers, the customers are human beings too...