I accidentally tested my max HR one night a few years ago. I was out training on my local street, after 10:00pm, and a car came from the opposite direction. As it approached, it started to slowly veer towards my lane. The road was quite wide, and I was on the left side of the lane so didn't really feel in danger, just surprised that people don't pay attention to what they are doing.
Anyway, just as the car passes me, I feel something hit me. A second or two later, I realised that someone from the car threw an egg at me. I was slowing to stop to check the damage, and suddenly had urge to chase. If only just to get a rego number. So u-turn, and I hot tailed it after the car.
I had already been riding for about half an hour, I was well warmed up, and the adrenaline was now pumping.
I only gave chase for about 40 seconds, before I settled down and gave up the chase.
Below is an extract from the ride, and you can definitely see the HR climb and plateau at 190 (the red line), before it started to drop.
A few months after this I started racing (amateur of course), I upped the training, and it was pretty common in the final sprint of a crit that I would peak between 190 and 192. Never more than that.
This was a few years ago now, and I only raced for a couple of years. I don't think I've been over 180 since.