In the years and years of recording my personal HR data (I think I'm on the 6th) with my beloved Polar 520, I have never had this happen. If it were not for the fact that the warmup and the second cyclocross race of the day yesterday at Quadcross had HR data that looked more "normal", I would suspect this was an error in reading. In fact, it still may be. One thing is for sure, that is one strange graph. I can't tell if I was truly going all out or what but it should rate an A+ for consistency I'd think.
I can say that I felt like I was going hard and than I really didn't have a whole lot more that I thought I could have given. It's odd though that I never recovered, not even a little, throughout the course. I still think something must have happened and the graph must be wrong. Who knows. I'm still alive though, even though I "flatlined" (at 168bpm which for me, is pretty darn high) for a good chunk of time yesterday.