I've got a couple! Sometimes common sense fails . . .
1. Not to eat an entire Monte Cristo sandwich at the Blue Bayou, then go on California Screaming immediately afterward.
Normally, lunch is followed by a break/nap back at the room so I can save half in our mini-fridge,and also digest the half I'd already eaten before going on big rides. One day we had already checked out since it was our last and we were headed home that evening. Seemed a shame to waste the food, so I ate it all! Luckily nothing tragic happened, but I was not a happy camper for about an hour or so after the roller coaster. Took me a couple years before I could eat another Monte Cristo.
2. Leave the Heelys at home.
They're banned now anyway, which is probably a good idea. About 6 years ago I was wearing mine while on a trip with friends. Coming off of Thunder Mountain headed towards Tomorrow Land, I was following my friend who was also wearing her wheels. The ground slopes nicely right through here so we had some good speed going. (Few minutes past closing time, so it was pretty deserted. We never used the wheels if it was crowded.) Just before the bridge, I hit gravel or something so my feet stopped suddenly but the rest of me kept going. Crashed
spectacularly. (I'm 5'11" tall, so when I fall it tends to be somewhat dramatic) In front of a family sitting on a bench, who probably thought I was less than sober since I sat there giggling idiotically for a minute until the adrenaline wore off the pain kicked in. Got some good road rash on my hands, tore holes in the knees of my jeans, and totally jacked up my left elbow when I tried to catch myself. It started throbbing and swelling and the muscles would spasm if I tried to straighten my arm. Another friend (daughter of a long time nurse) made me a sling out of ace bandage when we got back to our room. It was a rough night, lots of ice and advil, and loooong drive home the next day. Got checked out by my doc, he didn't see anything in the x-ray but said I had possibly cracked the head of my humerus. Yay me. And no, I didn't think of suing. That was completely all my own dumb fault.
Wish I could blame it on being a young kid, but I was 23 at the time.

Never took the Heelys to
Disneyland again, even tough its reeeaally fun to have them there. Even if they weren't banned, my income is pretty much dependent on my arms and hands, gotta be responsible and keep them injury-free. Being a grown up isn't always everything it's cracked up to be.