The Appellate Division recently addressed a situation where a guaranty accompanying the original lease was enforced post-modification of that underlying lease in 4 USS LLC v. DSW MS LLC where the court ruled that the Guarantee could "recover under the terms of the lease, prior to its modification".
This means that where you make a deal to modify the terms of your lease, you may still be personally liable under the old guaranty for past breaches of that previous lease.
The takeaway from this case is to address the guaranty documents in the modification documents and expressly state the intentions of the parties as to past breaches and their enforceability post-modification.
Real estate agents often only think about getting the functional results that their clients expressly state accomplished - this case is a reminder of the necessity to clean up the past before emerging into the goals stated by your clients for the future.
Otherwise, the past can come back to bite you.