What 531 said. Overhead power-lines seem to be the worst offender. The problem is made worse if the transmitter isn't making good contact with your chest, transmitter has low batteries, riding in cold weather (I think the cold weakens the batteries, or maybe you don't sweat as much, causing the contact problem.)
2003 Specialized Hardrock, 2004 LOOK KG386i, 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1
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).
Lucky for you it's all interference. Or if you think your HR was that high then better go see a doctor. I get readings like that all the time. Most common is a max cadence of 200+ which is just not possible....
FWIW...I had a Polar that would consistently register my max at either 232 or 235 from time to time also. I ended up returning the receiver for a replacement and have never had that happen since, even along the same routes as when it would consistently occur before. I don't know if that first receiver was just overly sensitive to electrical interference or what, but it got to the point that it was happening just about every ride. Polar was way cool about it and sent me a warranty replacement without any question. So I also figure that they had seen the problem before.
The first time that it happened though I thought my heart must be about to explode.
It's not correct. Mine does that sometimes on a downhill where the front of my jersey is flapping against the transmitter. If I press on the transmitter with my hand it goes back to normal. It used to happen more when I first got it and used to wear it too high on my chest so it wasn't making enough contact. Now that I wear it a little lower on my ribs it works better.
I once thought powerlines on a certain street were causing it, so I rode back and forth in the area of interest both directions on both sides of the street. The area had a dip in the middle, and by the process of elimination I discovered it was the downhill flapping that was doing it. Your results may vary.