If you want a very simple demonstration of GPS error just put yours down some place and leave it for a while (the longer the better for more variation). When you return you can see how far it has "moved", check max "speed", and review the track. Very interesting and instructive.

Also, if you wear a small GPS like I wear my 401 on my arm, be aware that it adds arm movement in with whatever is moving your entire body which is what you mean to measure. It can be a very big difference as your arm is capable of pretty quick movement. I have several witnesses that I have achieved 132 mph on snow skis just before wiping out (big, fast, arm movement) but it makes a great story. So, like any other useful tool, you need to be very aware of how it works if you want to use it most effectively.

I am a natural skeptic. I have a Wave and I have several GPS units. I would have to experience that speed while sailing a Wave to believe it and even then I would doubt it and look for a more likely explanation. Good story, though, and that's all that really matters!