http://speeddemosarchive.com/Left4Dead.html#IL

Originally I just improved this level by a couple seconds. Much later, coolkid showed me that it was possible to bunny hop in this game and so I was curious to try it out myself. Unfortunately the jump timing is very strict, down to a few frames, and you can't spam jump because there's a max input limit that will prevent you from jumping at all. Anyway, bunny hopping is doable in this level since it's just one open space with minimum RNG, and it saves 4 seconds which is a ton in a short level like this. I'm also using a glitch in the unpatched version that makes all bots spawn in the same place so I can kill them in two shots, and as a bonus it makes you able to move during the opening cutscene. If it wasn't for that cutscene skip I would have lost my sanity a long time ago, since I've done around 1000 attempts to get this time.