muscle memory-loss

1. When you know how to do something really well, but when you try to show people you suddenly can't seem to do it.
2. When you can do something without thinking about it, but when you consciously engage in said activity, you constantly fail at it.

I tried to show off to my friends how well I could ice skate, but I was struck by an acute case of muscle memory-loss and I fell on my ass. They wouldn't believe that I was ever good at it afterwards.