Did you happen to get passed by an ambulance during your ride? This is what happened to me this weekend. My HRM monitor (Polar S725) showed a max heart rate of 230 something, a cadence of 192 (I max out around 140ish usually) and a max speed of 67.5 mph (oh yeah, flying on those flats!). After downloading the data, it was obvious what happened (one single spike in my graph).