I posted this here instead of GAD because it is something that happens to everyone. When you think of something clever or funny or sarcastic to say in response to a situation and it is too late to say it.
Post comebacks here.
eg
When you are a student and your parents say
"don't they teach you anything in school these days?" you could say "no, they changed the cirriculum, we play tiddlywinks now"
or if your parents say
"if your friends jumped off a cliff would you do it too? you could say " I would think about it if there was a bungy cord attached"