It is often said that a particular contractual obligation has been waived so that a party can succeed in the teeth of what, apparently, he agreed to do.
In fact there are strict requirements for a true waiver to occur and even then the effect may only be temporary. The following is a quick summary of the relevant principles.
First, a waiver may be written or oral or implied by conduct. The difficulty of proof of course increases the less specific the evidence relied upon.