I don't know if this is a real slip-up, but it's definitely something blaming the medical guys of the House-crew!

In Season 2, Episode Deceptions House injects himself some meds. The scene begins with House sitting at his device, applying the vein-blocker, filling the syringe. Then he injects the med, pulls the needle out of his arm, draws his arm back to the body (to squeeze his armpit), waits, injects the second med, removes the vein-blocker and start opening and closing his fist.

Making an injection like this is one of the easiest ways to commit suicide!