On one of my local routes, I would crest a particular hill and my Polar HRM would indicate 275 BPM. This was obviously not right as a quick carotid check indicated. Yet it happened every time I passed this particular point. Eventually it dawned on me that there was a microwave repeater tower that came into straight-line view at that point on the road. At some point, I upgraded to a Polar HRM with the "Own Code" feature, and I never saw the spurious reading again.

This reminds me of another incident with the first Polar HRM. I was riding in the Ore to Shore Mountain Bike Epic in 2001. A portion of the route was along a section of power lines. At a rest stop/fueling station on this stretch, I was receiving a banana from a volunteer when I noticed my indicated pulse was "0". I remarked to the volunteer, "Hmmm... I seem to be clinically dead." He replied, "Sometimes you're better off that way!"