Another embarrassing person here!
I "ruined" my best friends' wedding (according to my mother, everyone else thought it was funny) because I started laughing about two minutes in, and literally couldn't stop. Everything was funny. The preacher kept repeating himself, the ring bearer kept touching my butt, a bee was divebombing the bride, you name it, it made me laugh. I looked like I was having a silent seizure, shaking head to toe, tears just dripping onto the floor, face screwed up like I'd tasted a lemon. The best man and I were both doing this, and feeding off each other. Everytime one of us would get calmed down, we'd make eye contact and get tickled all over again. When we got up the aisle after the wedding, he and I just HOWLED. I laughed so hard I wanted to throw up.
It seems I have this problem when I'm with certain people, too. I was at a college graduation Friday night, and it was the longest ceremony EVER. All the graduates were textmessaging people in the audience and swapping camera pics. I took this one of me and my friend Scott in the audience and sent it to the guy who was graduating:
Then I sent him a picture of his mom pretending to smoke a cigarette while looking at her watch, and a picture of my cleavage. (I won't post those here!

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When he saw them, he got tickled, then we started laughing, and we were literally sobbing with laughter, then everyone around us started laughing and eventually our whole section ended up getting shushed by the speaker.
Laughter is contagious...you can't be the only one laughing!