The elusive distinction between waiver and contract modification has reared its head in Massachusetts. What's the difference? A party who "waives" a contract term can retract the waiver in the absence of the other party's detrimental reliance, whereas a party cannot unilaterally retract a contract modification. Stating the legal consequences that flow from each event is easy. Figuring out which event has occurred is not.
Michael G. Hillinger, Waiver or Modification: That is the Question, 50 Boston B. J. 6 (2006).