haha, that happened to me with mapmyrun--it was always off a bit, but I used it to see how much time total I was running. I remember last spring running 12 miles and it said I ran 28, in just under 2 hours!
My phone did that reacently. I use RunKeeper and the GPS must not have had a strong signal, and it made my pace way faster than it really was. It was kind of nice, haha!