Clear Safari's cache and start private browsing mode. Go to YouTube and watch a few videos. You don't have to watch the whole thing, just until the entire video is buffered. Now go to the "/var/folders" directory, either via the Terminal or the Go To Folder option in Finder. There are a couple of layers of folders with random names, but search around until you find a folder with Safari in the name, then go in there. What I've found are entire youtube videos saved to disk. Privacy issues aside, Safari is writing gigabytes of video to my SSD for absolutely no reason.