I can answer this for Windows and Mac:
Windows: Find your minecraft folder (search: %appdata%\minecraft) and delete the folder named "bin". Re-open Minecraft and it will re-download the bin, fixing it.
Mac: In Finder, go to your Hard drive (the silver rectangle that is named "HardDrive", users, your user name, library, Application Support, Minecraft, and delete your "bin" folder. Re-open your client and it will re-download the bin, fixing it.
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The most popular way to fix your minecraftis to delete your bin, once your bin is deleted, your minecraftwill re-download,and you can continue playing, to do this on a mac you must go to, Finder, Library, Application Support, minecraft,in minecraftyou will find a folder called bin, simply delete and load minecraftagain. To do this on windows, go to Start, Run, type %appdata%and press enter, open minecraft,and delete the bin folder, and easy peasy,open minecraftand your done!
because n1gg3rs
it is bad
it wont you need to download the server thing of the minecraft website and replace the old one with it.
only minecraft classic multiplayer is free, beta mp is for premium only a.k.a buy the game.