bad wheel
one of the wheels on my outdoor hockey skates wore funny and got slightly less than round. which quickly made it very much less than round. at speed it would make an unpleasant vibration that shook my eyeballs. it eventually made my knee really sore. sore like i thought i did serious damage to it in a hockey game. but no. i rotated out the bad wheel. and my knee's all nicely healed up now.