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I admit this is only my first playthrough. I'm playing on a fresh copy of Left 4 Dead, so the only mod installed is your awesome I Hate Mountains campaign. This is also a relatively fresh install of Windows XP. I was playing through your campaign with bots, so perhaps this bug doesn't apply, because you don't intend your map to be played solo.
On the 2nd map of your campaign, I successfully called in the helicopter. I went down to it, and it crashed. I admit, you had me there, and that's a twist Valve never would have pulled. I approached the crashed helicopter from the left side (going toward the fence the helicopter crashed into, closer to the building I just left.) I then got stuck, literally, as in my player character couldn't move. I have some screenshots, which will hopefully help out.
I realize that this is your hobby, you're just three guys who sunk a lot of time into this, blah blah blah. So I understand if this is something that only happens with bots, in which case, this is the end of my thread. I only hope to help out, and regardless of what you decide to do with this information, I bear no malice or hard feelings toward you all.
Anyway, I hope the first two screenshots show you where I got stuck, and the last screenshot is some console messages about the bots being stuck. Basically, I and the 3 bots got stuck, and we were only able to turn around. The third screenshot is back in the safehouse because I wiped.
Hope those links work.
P.S. Thanks for the campaign. I'll try it the way it was meant to be played later (with real human beings.) Also, I read some other posts in this section, and I must say I did not find myself getting lost at all. For the first stage, I was able to make a beeline to the safehouse. Did not take a single step in the wrong direction. The openness certainly feels more realistic than the spoonfed "openness" of the Valve campaigns.
Of course, I'll try to replicate this stuck situation again. If I can't, I'll assume it's just a fluke or something done incorrectly on my part.
My apologies. I see this bug is already well-known. Search before posting, search before posting.