It mainly happened to me online when I needed to do things quickly to avoid death. In the single-player game where things move a bit more slowly, it's not a big problem.
But, in something as simple as a d-pad, it shouldn't ever happen. It's that dinnerplate design. It makes it too easy to hit the pad diagonally, giving you a 50/50 shot at selecting the right weapon.
Or check out my new webcomic: http://selfcentent.com/