I know there are lots of examples of this but today...
Talking in playground pre-drop off with a parent about transexuals (don't ask because I really can't remember why)
Kewcumber "Hey don't be rude about transexuals. I used to be a man"
Parent "Well I know thats not true"
Kewcumber "how do you know? It could have been a very good result"
Parent "Duh! I know because you have a beautiful son"
Kewcumber "Oh yeah, right, I hadn't thought of that"
It took me about to two hours to work out what the niggle was in the back of my head saying that conversation had a flaw in it...
I genuinely thought "Oh yes she's right I must be a woman because I have a child" the whole mechanics of acquiring said child momentarily (for a few hours) deserted me.