When I played Zombie Panic! Source (as the name suggests, a Source mod) I would usually play on the same map, Harvest.
On the map, there was a segment yo had to blow a wall in the basement. You can't stand close to it, because obviously you'll get blown to confetti. So there are two places where you hide for it to blow - one has a place where zombies get through, another that does have one, but is activated after the tunnel is blown. Obviously, most people chose the second place.
Well around all of the survivors are there, and when the fuse is blown we run to the tunnel to see...
That three survivors had barricaded the exit shut. And ran away.
So while we're all cussing and cursing them, we're trying to break down the barricade. Easier said then done, when zombies begin attacking us from the back. We quickly run out of ammo, and all die. Why? Because three survivors who forgot about us, barricaded the exit to stop zombies, and ran off in persute of weapons.
That was one of the main reasons I stopped playing, the other being how annoying it is to be a zombie.