This happened to me once in UO. I made the mistake of posting my best house design to the building forum, and within a month, a (prominent) person on another server had copied it to the tile, posted it up as her own, and I started getting angry accusations of theft on my in-game bulletin board. Five years later in the "hall of fame" thread, said person is still getting credit for my design, and their plagiarized version still stands to this day. Mine, of course, I took down -- the accusations were mortifying and spoiled my enjoyment, never mind that they were wrong and that I was the victim.

But if you really want to get into IP pirates and vigilantes, you need look no further than the Sims!