I've played only played versus a few times, but now I'm coming across this sort of stuff more often. In fact, playing as a hunter yesterday afternoon, I saw the one bolded above: one of the survivors was huddled by that door bashing it. So I pounced - and, ha ha, killed them. They weren't even watching each others' backs.
But, yeah, lame. I didn't realise the infected could startle the witch...
#4208804, By Left 4 Dead Organise Online Games Thread.