Motorbikes, I used to have very little regard for my own safety and crazy friends. I nearly came off the back of a bike at 145MPH, I was holding on to the bars behind me and my friend was hunkered down, I was thinking 'don't give way, don't give way' then my right elbow bent, my leg brushed my friends and he put the brakes on. 'My god I'm still alive!, incredible' was my next thought. I've never felt quite alive. Can't count the amount of other times I've almost came off the back of a bike, might explain why I've never ridden a bike myself.
Looking back it was so scary.