Copyright is more Eric's thing than mine, but recently I encountered what seems like an ill-advised exercise of that right.
I like to use references to classic movies in my problem sets for my intermediate microeconomics course. In recent years, I have been adding links to YouTube clips from those movies in the model answers. One of these problem sets involves a retired gunslinger named Shane. In past years, I have included a link to a YouTube upload of the final showdown between Shane and Wilson. Even more fun then the scene itself is the flame war in the comments about the true reason that Shane leaves town at the end. (Is it because he was a killler, or because of the feelings between him and Marian Starrett?)
In editing the answer this year, I find that the video is no-longer available on YouTube, due to "a copyright claim".
Which leads me to wonder, why would the copyright owners want to do that? Surely, no-one is going to not pay money to purchase the entire movie simply because the final ten minutes are on YouTube. To the contrary, the link itself, and the comments flame war, if they had any effect at all on sales, would surely raise interest in the full movie. Maybe the owners feel that they need to take a tough stand against any uploading of their IP, in order to create a reputation, but I can't see reputation mattering too much in the YouTube game.
PS to all you Shane haters: Yes, I know the acting in the lesser parts is awful, and yes, little Joey is really irritating, but beneath that the presentation of the subtexts and psychological relationships are very sublte. The move is well worth renting.