In Rehoboth Mall Limited Partnership v. NPC International, Inc., (Del. Supr., July 2, 2008), read opinion here, (and revised opinion here), the Delaware Supreme Court interpreted a waiver provision in a lease to prevent a landlord from using it to refuse a second renewal of a lease. Most contracts have a "no waiver" provision that ostensibly allows a party to selectively enforce provisions–or stated another way, the landlord in this case reserved the right to enforce the penalties in the lease for default in the future even if those provisions were not always enforced in the past.
Delaware’s High Court determined that because there were no defaults during the first renewal period, the landlord could not reach back to the original lease period to deny a second renewal. That is, the court reasoned, the "no waiver" provision could only be applied to allow the landlord to enforce prospective defaults–as opposed to retroactively penalizing prior defaults that were already waived.