Your heart in their mouth
Zombies aren't always aware of your presence, and sometimes that can create the most tension, the most stressful holding-your-breath events. After countless mini-zomb attacks, our party had got cautious and twitchy. Stray shots had been accidentally loosed into friendly heads. Nerves were afray.
So, when we entered a new room to discover more than a dozen Infected, we froze instantly. They trudged around, obvlivious to us. Like this, the zombies are sitting ducks, but we were alone and some way from the others, and this many nasties could easily kill us all. So, heart thudding in our larynx, we inched slowly out of the room, closed the door behind us and crept away, nerves on edge the whole time. "You don't want to go in there" we said, simply, to our fellow survivors.
United we're lunch, divided we're appertifs
When you go down - either under the weight of zombie numbers, or because a Hunter is playing piano on your bloody ribs - you need your buddies to save you. And you will go down. The scariest thing, though, is being the one survivor left standing, as your bullish but fragile teammates head off on their own and succumb to attack.
You've only a few seconds to act. We were protecting a near-dead buddy, sending the others into a dangerous room first, when he was jumped on by a Hunter. Halfway up a ladder, we had to leap down and save him, then drag him away to safety. But before we could stand him up, the other survivors went down at the top of the ladder.
While we knew that downed survivors can blast away with their pistols, it was clear that the two colleagues overhead would soon be overwhelmed. We also couldn't bet that the survivor down here would be safe if we went to rescue the others. This are Left 4 Dead's scariest (and generally final) moments - working out who to save. Because you know that if you don't save anyone you'll be all alone, with only the greedy, hungry mass of killer zombies for company...