What I foundwith such fast lens like 50 1.4 was, that it's very easy to recompose a focus, which I blamed at start as AF error. I think a lot photographers use this technique when holding camera in hands and shooting spontaneously. Total DOF for 7d is like 19cm on subject 3 meters away (at shorter distances it's even less). DOF is relative - in area of 19cm subjects are considered to be sharp, but you find more and less sharp areas within this range. If you block AF by pressing shutter half having focus on ie central AF point and after that you recompose your frame, you have a chance to fall out of focus. I've never found 50 1.4 to misfocus neither on 7d nor 5d2, assuming of course there was light enough.
Missed shots happen to me in real situations at low light or recomposing but never with good light or on tripod.