Lots of great posts here!
I don't have anything on the same level as most of these awesome tales of embarrassment but I do have one which was a true WTF moment.
Me and my (soon to be) girlfriend was making out in my room at school. For some wierd reason, it ended up with me being completely naked and her fully dressed while giving me head. In walks a mutual friend of ours and I manage to cover myself with a blanket right before he see's me in all my unholy glory. So, what he saw was basically me and my gf sitting next to each other, staring out in the thin air, doing nothing, me completely naked and her fully dressed. I'm sure he didn't know what to make of it.
Another time, same girl, same place. There was a lot of people and loud talking in the room next to mine. As we started to have sex, the talking dimmed down, which I didn't notice at the time, but in retrospect I'm sure they were listening in on the action. Anyways, I end up having a rather loud orgasm and the very instance I cum, the whole room breaks out in wall shattering laughter.
The real reason I'm writing this post is to tell one of my friends story. Not a real sex story but close enough I quess.
Me and two of my friends (all males btw), was smoking some weed and doing N2O. One of my friends did 2x8g in one go and soon after exhaling, he gets this wild and confused look on his face. He then threw him self on to the floor and started to take his pants off, clearly trying to free his cock from panty prison. It was a hilarious act to behold, I've never seen anyone try so hard to get their pants off in such a hurry while apparently being unable to comprehend just how to do so. In the end he figured it out and just as he get's ready to pull his cock out, he's facial expression completely changes in a matter of seconds, from looking like a maniopsychotic freak to that of an embarrassed and confused individual.
He takes his pants on and sits down, looking rather confused, with me and my friend just waiting with anticipation for him to include us in wtf just happened.
Once he was completely down, which in the case of N2O, isn't longer than 5 more minuts, he was able to tell us that in his mind at the time, it would be funny, not for him but for us, if he were to pull out his cock and start giving himself a blowjob (which he most likely ain't able to do, due to physiological reasons) so apparently that was what he was going for.
Obviously, this intention of his, didn't make it any less of a WTF moment for us nor any less of an embarrasing moment for him, to say the least.