Overstrumming is a term used to describe the occurrence of strumming notes that don't appear on the fretboard. Overstrumming can occur most often in extended strings of notes.
Overstrumming can also occur when a guitar hero guitar controller strums multiple times after only being strummed once by the player. Also known as Multi-strumming, double strumming, triple strumming, or nubstrutting; (A derogatory term for the tactic used by newer players that modify their controllers in order to multi-strum faster songs without much effort.) This controller glitch can be fixed, according to some reports, by using a rubber band on a certain part of the guitar controller.