Rule 16 says that a schedule "may be modified only for good cause and with the judge’s consent." This rule comes up any time a party wants to do something after a deadline set in the scheduling order, which is one of the more common litigation issues.
Parties will often, for example, let the deadline to amend the pleadings pass by, only to later realize that they want to assert an inequitable conduct defense (defendants) or wrap in a related entity (plaintiffs).
Good cause requires diligence, and in practice parties often frame the diligence discussion …