person A: Man I feel so bad about last weekend :/
person B: What happened?
person A: I went to the bar and forgot to tip my waitress
person B: Oh no.  That is pretty bad.  Do you know who she is that you could go and leave her a tip? Mistakes happen and there's not much you can do.
person A: Yeah she's there all the time.  My tab was pretty high and I was drunk.  I just forgot to put down an amount in the tip area.  Next time I see her there I'll float her some cash.