I've had my Garmin ETrex handheld show me doing 32 knots...while rolling my boat 20 yards down to my beach.

If I tack quickly, sometimes it will show a momentary speed of 25kts or such, which is obviously wrong.

I think when you bounce the boat around, as in rolling it to the beach or tacking in waves, something is screwy in the reception, causing a momentary 'jump' in position, and thus a speed spike, when it tries to figure out how you got from over there to over here so quickly.

The fastest I've been able to 'see' and believe to be true, with my handheld, is about 19kts. on a broad reach in about 20-25 knots wind, Uni, no spinnaker, boards way up, on my F16.