On the c.nerd.nu server, I was banned for griefing. I don't normally grief, but when I do, it's on creative servers, and to people I don't like.
For Vindicraft, I was just strip mining to find diamonds. When I finally found a pocket of 5 diamonds, this mad admin said in chat, "You know, xray is not allowed on this server." I figured he wasn't talking to me since I wasn't xraying. Then about a minute after i continued strip mining, I found iron and coal together in a clump. This apparently convinced the admin I was x-raying. -.- That was the stupidest ban I've ever seen.