Losing all of the skills you gain. No matter how good you get at something, after a few centuries you'll have lost your edge. You can also only practice so many things concurrently without giving something up. At some point, years down the line, you might try to ride a bike again and completely fail to do it, or try to sing and fail to hit all the notes that came easily before, or do gymnastics but the muscles you need are underused. It doesn't matter that you spent years mastering every skill, your abilities will degrade over time. You'll never really be able to feel sure about your own abilities except for whatever you've done most recently.
But the famous thing about learning to ride a bike is that you don't forget, even after decades. I've just looked it up to double-check and all I got was articles about why you never forget. It's like saying you'll forget how to walk up stairs or something.