I know there are people who not necessarily know about these, and they sure as hell make life easier.
First, there's the issue of the two clipboards existing. One primary, which saves everything you mark (not dependant on X), and the other, often used by GTK+ and others, where you copy and paste text using more widely used methods, mod+c and mod+p. I've noticed that if you try to paste text you marked, then mod-copied, and then try pasting it by usign the normal middle-click, it won't paste, because the ctrl-c actually overrides. Atleast, that's what frequently happen in my experience.
