I can confirm that this still occurs on a Mac Mini 320M running Lion 10.7.2. I am looking into this now and will hopefully get us some help on this issue.
For Windows, this issue would happen if you joined a server with any character other than the base 4. I believe that has been fixed for Windows, and it does not affect the issue with Macs.
[EDIT] I have found a solution for now, it is an issue reading downloaded mutators from out of ~/Library/Application Support/Killing Floor/Cache. This workaround was from a previous suggestion posted here -
http://forums.tripwireinteractive.com/showpost.php?p=579351&postcount=16 . Here is what you will need to do:
1. Make sure there is nothing in the folder at ~/Library/Application Support/Steam/SteamApps/common/Killing Floor/Cache, it should only be populated if you copied files from a Windows installation or similar. These fixes won't do anything if there are any files there.
2. Navigate to ~/Library/Application Support/Killing Floor/Cache
3. Open Cache.ini, take a note of the maps and mutators listed there (its the only way to figure out what the hash filenames correspond to)
4. Download any of these maps and mutators listed and place them into ~/Library/Application Support/Killing Floor/System instead.
5. Delete everything in ~/Library/Application Support/Killing Floor/Cache
6. Launch the game and join a server/go solo
Please note that this issue only seems to occur with some mutators (I believe INeedBodies is one) but I can't pinpoint it exactly so this is a catch-all if needed. Feel free to try using the cache for downloaded maps and mutators to see if there is any specific one that causes the error.
Hopefully we'll get a permanent fix for this sometime soon.