I have a couple good ones.
Humorous, yet embarrassing: I took a dump on Santa's knee when I was six.
Our out of town friends had come over to stay for a Christmas party in a few days, and they always brought the best baked beans ever. I mean orgasm in your mouth good. My mom would stash 'em away somewhere in the fridge like Easter candy for fear that they would be gone before the party even came. Naturally, I used my superior child intellect and located them. I devoured a bowl or two, and put them back as best I could when I heard my mom coming in the door.
In about an hour or so, we went to see Santa for wintertime jollies. We apparently came too late, because the line stretched to an ungodly length. To this date, it's the longest line I've ever seen in my life, including Space Mountain.
Of course, the beans start kicking in, and I'm letting loose farts every time I do that line waddle. I try to hold them in, but they keep coming, and we keep getting closer to Santa.
Fastforward through two and a half hours, we're at Santa. I'm still farting. Luckily for me, this time has gotten me reacquainted with my intrinsic ability to pass gas quieter than a mouse. You couldn't hear them if there was a megahorn up to my butthole. People start shooting glances at the pudgy kid behind me. I feel kind've bad for the kid, but he isn't paying attention to anything but Santa.
I get let up to Santa. I try to get myself in a good position, but this guy is intent on taking my anal virginity with his knee. I wonder quietly to myself how a pedophile could get a job as Santa, but I let it slide, seeing as he is Santa. We're making small chat, and I feel some gas coming on. I panic, but remain calm on the exterior. I inch it through my bowels like I've been doing for the entire wait in line. No problem, by the time Santa gets a whiff, he'll think it's the fatty behind me. Just one more push and...
Plop.
I have a turd the size of a monkey skull sitting in my whitey tighteys. It's hard enough that it won't squish out of my pants, by it's warm, and Santa feels it. He goes pale as a ghost. I do too. I'm terrified. I'm getting coal this year for taking a dump on Santa. A helper elf comes over, and relays the message to my mom sitting five feet away. Her eye starts twitching.
In retrospect, the guy was very professional about it. He fastforwarded through the standard Santa fare, got me off his knee, and kept the line moving. He started sitting everyone on his right knee, though. I had to tell mom about the beans (because really, they were the least of my worries at this point), and I get a stern talking to by dad. We have a laugh, and everything is okay.
My parents never touch the situation, since it embarrassed them as much as it did me. However, those out of town friends bring it up every Christmas. I have my own stories of the husband (he once stole a pig and woke up with it in his bed when he was in nothing but his boxers), and we've made it a tradition to banter around the fireplace on those cold and snowy eves.