"people riding bikes using jerseys made of synthetic material, especially under low humidity conditions. the two pads on each end of the strap are attempting to record a few millivolt signal off of your chest. the wind rushing against the jersey causes a huge voltage gradient that confuses the HR sensors. I've found that using water/spit/gel doesn't fix this problem. A common scenario is that the HR looks reasonable, then I get up to speed and my HR goes through the roof, often up to 240 bpm. Then I sweat, and the humidity reduces the tendency of the material to create static fields. but then I'll glide down a long hill, and the HR zooms up again, defying all logic as I'm not working hard at all at that point.
In confirmation of #2, I once tried this experiment. Earlier this year when I had the problem a lot, I got ready as normal, but before I got on my bike I looked at my HR, which was something like 72. I lifted my jersey way from my chest with one hand, and rubbed the jersey with my other hand, vigorously, but no so much that my HR would climb much. My HR very quickly hit 240 according to the monitor.