Funny in retrospect, but it really had me rattled.
I thought my general chemistry class met at 11 am MWF, so I was reviewing my lecture notes this morning. When I came back from a bathroom break I saw the schedule I printed and put on my door showed my class start at 10 am. It was 10:20 am. I quickly grabbed my laptop, laser pointer, and sprinted out of the room, only to realize that in my haste to disconnect my laptop I knocked the battery out. I get down stairs and see another chemistry professor teaching in my room. "Oh fuck. He's covering for me. I'm so screwed." He looks at me and gives me a weird face. The only thing going through my mind is, "Fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck fuck FUCK!" At that point I poke my head into the room at which point the professor says, "I admire your dedication, but my class still has 20 minutes to go before your section."
At that point I went back to my office to get the other things I forgot for my lecture, and appearently visibly shaken. I managed to compose myself before class, which went really well. But for 10 minutes, I thought I flushed my career down the toilet in my first week.