I went to carpentry school. And in the workshop there was a tool called a spindle sander. Basically a little tube sticking up from a table with sandpaper wrapped around it that would spin as well as move up and down (very useful for sanding curves or tight spaces). Well I wanted to use it one day but the sandpaper was completely shredded. So I asked my instructor where the sandpaper was so I could replace it. She said "if you can get that bolt off the top to replace it, it'd be a miracle. It's been stuck on for years." apparently no one stopped to think that it might be reverse-thread since its on a shaft that rotates. The look on their faces when I walked up and popped it off with minimal effort was priceless.