This happened earlier today. So I was in class listening to a lecture and it dawned on me that I couldn't care less about what my professor was talking about.
So I decided to start holding my breath. Now, this is something I normally do if I get really bored. I'll hold my breath for as long as I can and then I'll rest for 3 minutes.
I'll do 5 sets of that. I also make sure to pinch my nose as to avoid cheating. I've actually gotten pretty good at it and can hold my breath for 2 minutes and 15 seconds now.
Anyways, I start holding my breath and I'm feeling particularly good and I start thinking I can break my record today.
I got to about 2 minutes and 20 seconds on my first attempt and then I decide to drop out. "I can do better, let's go for 2 minutes and 50 seconds".
So I go and I'm feeling super confident about it. I get to 2 minutes 20 sec and I'm feeling a little light headed. Get to 2 min and 30 seconds and I'm struggling.
Like I'm thrashing around silently in my chair, but I'm going for gold. Get to 2 minutes 40 and I'm seriously hurting. Things are getting bad.
I close my eyes to avoid looking at my watch (It also made the seconds go faster) All of a sudden I open my eyes and I'm looking up at the ceiling and everyone is flipping out.
As I learned from the girl sitting next to me, it turns out my maximum effort resulted into me passing out, smashing my head on my desk, and falling out of my chair.
I spent the next 10 minutes trying to explain to everyone that I was fine. Lecture finally resumed and I just sat there feeling like a dumbass with a gnarly headache.
Lecture still sucked after that.
TL;DR Tried going for Olympic gold in breath holding. Failed and ended up spread eagle in our lecture hall.
EDIT: A word