I have a Canon T3i (600d) and two lenses: EF-S 18-135mm and EF 50mm f/1.8.
While working with manual focus, I can shoot out of focus (on purpose) with the 18-135 (as expected). But with the EF 50mm, it will only trigger when the subject is in focus, otherwise it tries to "focus" (I hear and feel the focus motor, but as the focus-ring is deattached, nothing happens).
It's weird, it's supposed to let me shoot at any focus (as with the 18-135). I don't think it's a camera problem.
Is it a problem or a specification of the EF 50mm f/1.8? It works perfectly with autofocus.