I [think] never found an editing mistake in my videos yet, but today when I uploaded a video I forgot to set the thumbnail, so it came out with an awful one. When I went and changed it, it's still showing the s****y one! I'm p****d off and don't know what to do hahaha
It saves the thumbnail in your cache. If you clear your cache (or check from a different browser) it should show the updated one. If it doesn't, re-upload it and clear your cache again. That should work.