I use a Garmin sport watch in civvy mode. Normally take it off before a punt and leave it in the car or at home.
About halfway through a punt and I'm shagging away in missionary and my watch starts going berserk. Turns out it's got a function that detects abnormal heart rate. I had no idea how to stop it so I had to quickly take it off, but it carries on squawking, presumably it thinks I'm now dead. Had to turn it off completely. Bit of a mood killer to say the least.
Explained it to the Thai girl I'm with and she tells me that her younger brother had died of a heart attack in his 20s and she's now visibly upset. So just chatted to her for the rest of the meet.
Safe to stay the watch doesn't come out punting any more.