Well for music they can sure detect them because its automatic. For images small channels wont have problem because i guess them pay people to watch videos of bigger youtubers.For random flash games, you have to watch out for music and images imho. The could be using images taken from google, even things like textures, so you never know. Although, I don't know if youtube will be able to detect them.
Yeah for most games it's the copy right music in it. And some game companies don't like the cut scenes shown. But just maybe do a little research about if a game your gonna record has had copy right issues before with the company and why. Like for one I don't do any of the Saint Row games even though I'd like to. Or I probably won't do anymore tropico games either.