I've done Versus on No Mercy heaps of times and that's never happened. Only real "glitch" I've seen is on the rooftop, the music for the second tank started up but it never appeared, meaning the helicopter never appeared (which screwed us over as survivors as the infected just kept respawning until we were dead, and this has only happened once). Also another one is having the survivors crouch under one of the ramps making it very hard (though not impossible) for them to be attacked. Oh, and on Death Toll on the first map, having all the survivors jump off the cliff at the spawn and still getting a score of about 200, which is more than you would get if you got through half-three quarters of the map.