I used to mount a Garmin GPS on my handlebar when measuring, just as an additional check of distances. However, I stopped carrying it after I noticed it would occasionally show the distance traveled as shorter than what I'd actually measured. I discovered that it would sometimes loose satellite contact and later pick it up again.
I believe that when it did that, it treated the distance between loosing and regaining satellite contact as a straight line, which was always shorter than the distance that I rode. For somebody using a GPS during a race, this would certainly report the course as being too long.