Maximum Heart Rate Error?

Have you tried taking your pulse manually and comparing to the Watch's reading? (My maximum rate is about 35 beats higher than the "theoretical maximum", and I'm 68.) When I'm doing intervals on the treadmill I can use the treadmill monitor to check the Watch--they're often off by a few beats, but generally pretty close. Occasionally, when my arms get sweaty, the Watch is obviously wrong, showing only half the actual rate.

I regularly get up above 180bpm without trying hard (37 years old). So that theoretical limit is not accurate for everyone. Check your heart rate yourself via some other means and do a comparison. But in my experience it's not unheard of to have a high heart rate during exercise. I was able to get to 210 one time after sprinting (using a different heart rate monitor).

This happens to me too.

I have been tracking my rowing heart rate with a chest band for some time, so I know my heart rate never jumps to over 200 for a couple of minutes.

Looking through my heart rate data on the iPhone I have found one or two unusual peaks like this. You can delete these rogue data points if you like.

I have had my Watch for a year and it regularly reports really low heart rate readings (in the mid 40s) and then changes within seconds to a more believable value. It has frequently shown high rates like 169, 200 when I'm walking up hills. I'm 68 and would expect 130-140, which I do see but my M.D. told me that if my heart was beating at 169-200 I would notice it. My left wrist is hairy and I sweat in warm & hot weather but I got high readings when walking this winter in cold temperatures.

I just put the Cardiogram app on my iPhone and it's Watch counterpart and it is showing pretty much the same a the Health data which makes sense since there is only one data source. I'm going to try wearing a Polar bluetooth chest strap and see what it shows.