Generally speaking, many contracts have what is called a zipper clause. The clause says that the contract cannot be modified without a signed agreement. This puts barriers into the path of modifications, but requiring the parties to come together and agree on the change. Perhaps delivery on Monday is not working out, but delivery on Wednesday would. A signed writing modifying the delivery clause changes the underlying contract and the parties continue performing under the contract as modified.
Perhaps no zipper clause is included. This allows the parties to modify the contract in a variety of ways. For example, the modification could occur by oral agreement. Jane could call up John and request the day of the delivery be changed. If John agrees, the contract could have been modified.
More likely is modification by actions. Testing this kind of modification is based on the course of conduct in performing the contract. Say the written contract says deliver is to be on Monday, however, every time delivery has been on Wednesday. If both parties accept this deviation from the contract, eventually the conduct of delivery and acceptance on Wednesday modifies the contract to this different clause.
This may not be much of an issue. However, it could be a significant concern. For example, an operating agreement is defined as a contract between all members. If the operating agreement is modified by conduct, it could appear as though the members are not following the operating agreement. Should litigation happen, a court may be faced with the question of why should they enforce an operating agreement the members don’t follow.
In other cases, modification could be argued as proof or defense against a breach of contract. If delivery is supposed to be on Monday and there has been no written modification, one party may attempt to argue the other side breached the contract by delivering on Wednesday. The Court would have to look at the facts to see if the contract was modified or if improper delivery was grounds for breach of contract.
Modification happens fairly frequently, as modification can be by conduct it could occur by accident. It is always important to understand how your actions can impact your contract to guard against having to perform in a way contrary to the underlying contract.