OK, this one hits close to home. God I hate that. My story is almost as stupid. In high school I was a pizza delivery guy (no, no naked ladies ordering pizza, sorry...). I went to one house and rang the bell. The lady opened the door and told me she'd be right back. I went to walk into the foyer and didn't realize they had a full length glass door in front of the main door. Me and my pizza walked right into it. Luckily I didn't break the door or dump the pizza. Just felt like an ass.
And my second story was funny because it didn't happen to me. In my high school, all the doors were glass. And usually, next to the glass door was a full length glass window. Made for nice looking entries. Well, one day we were screwing around and threw a bunch of physics equipment out the window. But feeling guilty we decided to go gather it all back up during a class break. So a small group of us went running down the stairs to the exit. For some reason, the guy at the front thought the glass window was actually an open door and ran full speed into it. It was that thick reinforced glass so it didn't break. And we all had the pleasure of seeing an actual human being bounce. It was the funniest freaking thing. That boy wasn't right the rest of the day....