I got out of gym for a full year because I went to a doc at age 13 and convinced him that my knees weren't growing right! He wrote a note dismissing me from gym participation for a full year, saying I had some bone-growth-thingy problem, and had to be careful walking up and down stairs!
There was absolutely nothing wrong with my knees, and he darn well knew it. If I could meet him now, all these years later, I would kiss him.