In this day and age ... and in the USA to boot, you would not expect for such a mistake to occur ... but it did.
The Harmon Hotel (part of Las Vegas' MGM City Center Project) may have to get its height cut by more than half. It's currently at 24 of 49 proposed floors. But someone messed up and used the wrong rebar in construction. So it looks like they will have to put a roof on it and call it done. Current rebar will hold 24 floors, not much more tho.
http://www.vegastodayandtomorrow.com/images/citycenter_081205k.jpg The building with the multi colored blue windows.
The only other thing they could do is implode the current structure and start over ... economy says no. Fun.