On my long run you may recall my HRM wigged out and I shut it off.
I found out why. I had replaced the battery two days earlier and I neglected to install the tiny O-ring when I put the battery cover back on.
Apparently I sweated enough that the water caused the battery to short out (see the rust and corrosion that happened in just an hour on the battery).
I only realized this when I saw the O-ring on the rug in the bathroom, picked it up, and realized what it was!
Keep them batteries dry!