In left 4 dead i think i found a secret cut

during the subway level, at the start i was droped down the hole and i went through the little dead-end (the kinda alley, not the one with the stairs) and i jumped on top on the AC i shot at it and i went through it!

i ended up going into the vents and through near the subway, just telling u incase you dont know ;p

There was one time, i don´t remember what player, but i do recall he was nice and friendly, who take charge of the whole team, and took us trough the first level fast and smooth.

He knew two secret passages. One was destroying a closed wall hitting it like 20 times to open it, and the other one was a shorcut i never saw before (maybe that´s not a "secret"). I guess there must be a few surprises in the other game scenarios as well.