Is a fully executed lease legally binding?

Without further information, it appears you do not HAVE to execute a new lease... they have an argument (should they nevertheless honor the signed lease and let you into the apartment - which CLEARLY is your MAIN concern...?) that the $1400 was an inadvertent mistake and that YOU were absolutely aware that the rental amount for that unit was supposed to be $1450 because that was the information your were provided with in the application to rent the apartment. And if they successfully sued you on that basis (chances are that they wouldn't, but...), you could be liable for the additional $50 rent back to the beginning of the lease once the trial finally occurred and judgment was rendered. Further, if your lease is a month to month lease, once they let you in they can give you a 30 notice to quit for no reason at all, and they MAY be able to raise the rent after the fact... If you want and like the place you've found, you should try to work out the deal with them by agreeing to pay the $1450 (maybe after a few more months?) I presume your first few payments for rent (and deposit) were only $1400? In which case you have even BETTER grounds for keeping it as is and NOT entering into the lease revision...but clearly, now is the time to get something from them for your willingness to unilaterally execute a new lease... Good luck