Many of the big mods such as Rub, Grey Wolves, and NYGM change many aspects of the game. If someone who is just running 1.4b and nothing else tries to join a game hosted by someone with say Grey Wolves they will get kicked. The way around this is to either make a seperate copy of the game to take advantage of these mods in single player keeping a multiplayer copy ready, use Jones generic mod enabler to enable/disable the mod, or play with people who have the corresponding mod(s) installed.
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