Nothing bad happened on my worst experience. I bought a Ducati Monster 900 on a good buddy of mines recommendation. He was a road racer, and even raced at Daytona. After I got it, he asked to take it for a spin, so I let him.
Next thing I know, he is coming down the road at about 60mph and pulls a wheelie and rides it on the back wheel until he is out of sight.
I am just picturing the thousands I just spent tumbling down this country highway. I never let him ride it again, my nerves couldn't take it.