One of those lies they tell children actually did come true for me!
I have always enjoyed making weird faces in order to make people laugh. This often included crossing my eyes. I also tried to train my eyes so that I could make them go in opposite directions (unsuccessfully) just so that I could make freakier faces.
Now I have a lazy eye and therefore can no longer cross my eyes. I can't prove it, but I have to wonder if all the effort I made to be good at crossing my eyes was a cause of this. I might have stretched the muscle too frequently, thereby weakening it or training it relax more than it's supposed to.