The three captive phillips head fasteners on the bottom edge can get jammed easily. I'd say about 50% of the non-isight G5 iMacs I've opened have had at least one of them over torqued.
One technique that has worked is to apply gentle pressure to the lower corner or center, just above the frozen fastener and turn the head to the left (undo) with enough torque to move the fastener back into position. The danger is in stripping the head, so be careful.