There are different claims of wars extended by diplomatic irregularity which involve long peaceful periods after the end of hostilities where, for various reasons, the belligerents could be considered to be in a technical state of war.
For example, occasionally small countries named in a declaration of war would accidentally be omitted from a peace treaty ending the wider conflict.
Such "extended wars" are discovered much after the fact, and have no impact during the long period (often hundreds of years) after the actual fighting ended.
The discovery of an "extended war" is sometimes an opportunity for a friendly ceremonial peace to be contracted by the belligerent parties.
These "treaties" often involve non-sovereign sub-national entities, such as cities, who do not in reality have the power to declare or end wars.