Mistake made on my lease to my advantage (the renter)?

If the lease was signed by both parties, then that's a legal doent. The only thing they could do is ask you nicely to undo the mistake and initial next to the new date (the correct lease end date) that you and the landlord have agreed to the correction. WITHOUT your consent or signature, they are not allowed to evict you because they didn't catch their mistake. You don't have to play dumb - you can say you knew that but it was their responsibility to check it. A lot of landlords and realtors make mistakes when advising people to sign doents. If they didn't catch it, they have to hope the other person (to whom they have an advantage) is willing to recognize the mistake and to be nice and undo the mistake.