Garmin Edge units occasionally will record an erroneous position and show ludicrous speeds and/or distances. I don't think this is what happened in your situation. If you moved the magnet past the sensor quickly back and forth a few times when you were fiddling with it, you might have recorded a high speed...but who knows. Go to the Edge forums at Garmin to get more info and advice: https://forums.garmin.com/forumdisplay.php?219-Edge-500

Also, max HR depends on the individual, and it's extremely difficult to achieve. 180 is within normal for many people past 60 (I'm 60 and mine is 183). That said, HR straps to at times record erroneously high readings which can be due to poor contact with dry skin in cool conditions, pickup of static electricity from flapping jerseys, etc.. Again, the Garmin Edge forums offer more info.