Online video has spawned a spate of copyright violations. Many users show no compunction about uploading network TV shows, copyrighted music, or other content that they don't have the right to distribute. In attempts to appease advertisers and producers, YouTube is in a constant battle to keep copyright violating video off it's site.
If you're curious about what videos have been removed from YouTube due to alleged copyright violations, check out YouTomb. The site was created by MIT students and lists videos that have recently been taken down from YouTube. You can't watch the videos, but you can see titles, thumbnails, and information about who requested the removal. It's interesting browsing for anyone interested in copyright issues, especially as they relate to web video.
If you want to make sure that no one asks YouTube to remove your videos from the site, be sure to read our article, How to Avoid Copyright Violations, before uploading your video to YouTube.