I always tip, you never know if the waiter/ess is having a bad day or has personal issues bothering him/her. Leaving a nice tip would make him/her feel that much better.
I have been to restaraunts where they have forgotten parts of the order, or brought the appetizer after the entree. When you think about it, who really cares? Everyone makes mistakes. I am sure you would want a mulligan/2nd chance if you made one so let the the waiter/ess have one also.