I had a similar problem w/ Motion Based after a ride that I did on Saturday. Got an error message and then the GPS unit froze up completely. Wouldn't charge, wouldn't turn on, nuthin'. Did a hard reset and all was well. Except for my maximum speed of 68.1 mph or something like that. The unit was in my jacket pocket, so maybe that had something to do with the strange speed or maybe I went past something that messed it up.
Motion Based does some funky things with data sometimes, because Training Center seemed to have the maximum speed for that particular ride a bit more accurate (38.7 mph). I've only really seen this issue when riding on the road, but again I don't have a mount on the bike I ride on the road (calling it a "road bike" would be inappropriate), so I'm thinking Motion Based is calculating some correction when the satellite signal is interrupted from bouncing around in my pocket.