I've never had it happen with the heart rate but my Garmin Edge 305 showed a couple MPH spikes from 15 mph to 32 mph while riding on the flats in Florida. I doubt I could ride 32 mph going down Pike's Peak. I download my date into Garmin Connect and the old Training Center. The spikes show up in Connect but not in Training Center. I realize that makes no sense but that's the way it is.