Oohhh, I'm on youtube (for about 5 seconds).
Yeah, the end footage that Jackie posted was what I was talking about. He didn't show the part where I was shooting at the wall with my M4 because I heard the Boomer too, yet no one on the opposite team accused me of hacking. Jackie started shooting after I emptied my clip and managed to hit the Boomer, who was making a racket, and they started accusing him of using a map hack. My guess (and I'm sure everyone on our team guessed too) is that they were trying to set up to boom us as we went around the corner, but they waited too long. They made the same mistake upstairs too; the guy spawned too early, we heard him, and I think xpkoala killed him through the wall.
Jackie just plays L4D the way other people play CS; you know all the hiding spots in the map, you predict your enemies' movements, know how to handle your weapon, etc. He also usually plays on a team of people he knows, so his team already has a level of trust that a randomized team doesn't, although I'm pretty sure he could do equally well on a random team too.
Still, 7:00 looks suspicious. I call major hacks on you! I have Valve on the phone right now...