It's extremely unlikely; and I've never heard of any airline doing this on a RTW ticket. I'd put the chances at less than one in a million. However, it will be extremely costly if they do. They have the legal right to recalculate your fare as the sum of the fares for one way flights for everything you flew. All that said, I'm doing exactly this in a few weeks, -- not showing for the last leg of an AONE3.

Remember, one of the things you can do is to schedule that last leg really far out into the future. Who knows, -- maybe you'll have occasion to use it at some point before the ticket expires. You'll be able to change the date, time, airline and flight number of that last leg without charge any time before you fly it (assuming, of course, that the inventory is available). I'm not doing that this time because no changes are permitted on my ticket.