Copyrighted music...
Used to be the case that YouTube were happy to let you use most things (provided your epic was not for commercial purposes) and they would feature a link to iTunes and make a few sales fro the artist. A good feature I have bought a fair bit from iTunes this way.
Now, it seems Google are having a spat with whoever (again) and although they allow some music (with the iTunes link) it will not however show on a mobile device such as a smart phone or tablet! So you check out your piece of music with a test sample to YouTube and all is well, you edit your masterpiece, upload, only to find it is blocked for no good reason to half your audience!