I need to be at school at 3pm...DOH!

I had my head completely up my butt today.  I was screwing around all morning, waiting for my Humanities class to start in the afternoon.  I looked at the clock numerous times and even thought that it seemed kind of late to be starting class at 3PM. Well, it was kind of late. About 2 hours late. The class actually starts at 1PM. I didn't put together the fact that my math class starts at 3:30 and it would be impossible for Humanities to start 1/2 hour before then (it's a 2 hour class). I didn't even realize what was going on until I tried to go to the classroom and it  was the wrong class. I was utterly confused, thinking maybe I had the wrong room or even the wrong building. What a dummy. I had to email the instructor and tell her what I had done, hoping for some understanding. I hope this isn't a sign of things to come for this term.