The launcher does not leave me playing Offline!

*Every time I try to play, the launcher starts downloading some files and then inserts me into the game, but when I'm offline and It cant download, I receive an error message that says "Can not load 1.x.x.jar".
It also can not load modified versions, and when I try, it corrupts the file version, I can not even run it in the other launchers after that.
Now I'm temporarily using another launcher, until you fix the problem.*