Hi
First of all :bms: is do awesome.
I’ve got a crash when I was amazed on lucky pick of needed vent hole on big fan fly (bm_c1a4c)
I started to load-game and try another ones. Then I realised a portal behaviour… So, I ‘noclipped’ and fly to take a look. And, looks like it was in some actual portal place — I’ve got a crash, giggled, and I would never write about it, but they said to tell a programmer 
https://dl.dropbox.com/u/1846455/temp/BM/hl2_4104_crash_2012_11_28T20_40_4C0.mdmp
Keep it up, you’re the greatest (I’m serious, you are)
Source SDK Base 2007 (all original installs)
Win7x64
Core2duo e4600 2.4G
GeForce GTX 460
PS I’ve got some issues with an alternate ways-to-see in the game (you know like… wow “you have found a secret area”) so, when I took a look at different vents I saw plenty of red lights and only one, on the top, with bright lamp (which is unreachable, I even tried to fly up from the very fan, and just then I assumed to use a damn cheat). So first of all I decided to examine the wrong ones, if there is some goodies…
I play Black Mesa for a first time, slow and nice, checking every detail (cool details) So I hoped - one way is the correct, others are false or labyrinths, and maybe the hardest to get is ‘the secret area’ one.
…I’m sure plenty of oldskool gamers did just the same