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Broken speedo, broken cable, broken cable gear, broken cable drive gear, un hooked....to name a few. It is pretty basic stuff, all mechanical so it would be easy to figure out if you start taking it apart.
Speedo gears are made of a very cheap material. Metal upgrades are available. I've heard the actual gears inside the cluster are what cause this the most. Then again, this is all hearsay for me. I haven't cracked mine open yet but I will report more when I open my cluster this weekend.