Whenever you run a mod, if a client connects that does not have the mod the server will try to send it to them provided sv_allowdownload = 1
However if on the client machine cl_allowdownload != 1
Then the client wont download and they will show as being disconnected due to invalid pk3 etc.. etc..
For some damn blasted reason the default is off on the client side, and most peeps don't change it.
Thus you have lots of players trying all these great modded servers but wondering why they can't play. lol