I'm a Garmin retailer and have an unusual one; a customer's unit has an elevation display stuck at roughly 65,000ft (the actual number corresponds to exactly 20,000 meters, making me think it's the upper-limit Garmin allows). For a ride, it shows zero elevation gain.
I've done a hard reset, I've set the current elevation, and it's fixed for a few hours... and then gets stuck again. I did try the "chat" thing with Garmin Tech Support, but it displayed the "we're looking for the best person to deal with your problem" message for 6 hours before we closed the window. :-)
Thanks-
Mike Jacoubowsky, Chain Reaction Bicycles (local retailer in Silicon Valley, *not* the on-line place in Ireland)