I am currently having issues with my DayZ Mod for Arma2/OA.
What's basically happening is that the game will load with all the mods correctly enabled, but when I go and try to join a server, I get disconnected with an error saying that I have extra files, that I am missing files, or to the point to where it freezes at trying to make my character.
Best Answer
Without looking at the specific error, or mod you are playing it is hard to say, however this is most often caused by incorrectly signed files in the addons folder of the relevant mod.
This is usually remedied by reinstalling the mods, making sure they are installed correctly (especially with mods such as Overpoch that combine two) and connecting to a server you know to work.