I just set my HR max record at the end of my ride today. It went from 173 to 174!
I raced a segment for a couple minutes, and at the end I coasted for a while because of getting stuck behind traffic, but then did one more hard effort. My heartrate got to its max just after the end of the segment.
I had been riding above threshold for approximately 5 minutes before that segment. Earlier in the ride, I did an above-threshold effort for something like 15 minutes and I was feeling wasted on the way home.
Maybe it something about the periods of coasting right at the end that did it?