Similar thing happened to me this summer when my Garmin Forerunner reading was (gasp) 243. I thought it was a glitch, too, but it happened again on my next ride coincident with a climb and my HR stayed there for ~40 minutes. Long story very short, turns out it wasn't a glitch at all, rather, supra ventricular tachycardia for which i'm being ablated on 10/20. (It happened again during a treadmill stress test, making it easy for the cardiologist to make her diagnosis.) Hope your reading was, indeed, a glitch!