Since there are no cables or anything on newer bikes. I would not worry too much. The speedo is basically a voltmeter. probably just hung up at the pivot point. If it happens again, give it a good rap and see if it goes back to normal...
2011 XR Midnight Cherry, Hard Bags, Vic htd grips & seat, Vic medium Windshield, unlocked speedo, Lloydz Air filter, VFC III, Lloydz ATS +4, Vance & Hines slip ons
1993 Vulcan 1500 (sold)
1981 Yamaha Seca 750 (ridden into oblivion)
1977 Honda 550 (sold)