No doubt a few people have had this problem. I believe it's a game bug.

In order to connect to a server running the campaign, that server will need to set `sv_consistency` to `0`. If you're running a standalone SRCDS server on the internet you should already know how to do this.

If you're running a local server from your game, you will need to use the developer console. You can enable it from the game menu, and open it using a keyboard key such as ' or ` . When inside, type `sv_consistency 0`.

This console variable is normally used to prevent clients from using custom models and materials, so they can't cheat by making highly visible special infected or invisible walls. The campaign is probably using custom models for special infected, and even though you have those same files on your own machine because you downloaded the campaign, the game is getting confused. It's not a fault of the mapper.

If you're a server operator and you want to place a mapname config for each map in maps/cfg to disable sv_consistency on a per-map basis rather than globally, here is a list of the maps in this campaign according to the mission.txt of the first VPK. If this is wrong, please reply with corrections.

l4d2_deathcraft_01_town
l4d2_deathcraft_02_ravine
l4d2_deathcraft_03_stronghold
l4d2_deathcraft_04_nether
l4d2_deathcraft_05_lighthouse
l4d2_minecraft_evolution
l4d_deadline02_vs
l4d_sv_deathcraft_lighthouse
l4d_sv_deathcraft_town