It is likely to be one of two things, both fairly rare but known Tenere issues.

(1) There has been a momentary interruption in the dash's power supply whilst the engine was running (for whatever reason) which has left the tacho's timer circuitry 'confused'. The fix for this is easy: Disconnect the battery for a minute or two and reconnect.

http://www.xt660.com/showthread.php?t=25755

(2) Your tacho needle has come a drift from, or loose on its spindle; in which case at some point the needle will fall off!

http://www.xt660.com/showthread.php?t=25431

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