I once woke up on the day when I was supposed to fly back to the uni for the January exam session, stretched a bit, made a few sad faces about leaving home yet again, then thought "OK, so was my plane at 1:05 or 1:15pm?" and grabbed the ticket to make sure. Well, I checked the ticket at 8am and was already late by 30 minutes... I managed to catch a different flight, but it cost me a not insignificant sum.
And then there was another time, when I wanted to be smug when talking to my best friend's girl, who was from a small town (well, the village I lived in at the time had a whopping 17 inhabitants, so probably a bad idea)... So I asked her "so, do you get excused from school to help with harvest?"... Everyone looked at me with a slightly amused look and my friend said "harvest's in August"...
He still reminds me of that whenever I get too smug.