I went to a recentish Formula 1 exhibition at my national museum in New Zealand and the big draw card was supposed to be a McLaren MP4-21. It was in all actuality an MP4-20 tub and rear end, the front wing from the MP4-18A (by the looks of it) and Lewis Hamilton labeled as the driver on the side. A little misleading but I guess if it can be done there's nothing stopping them throwing their names on things.
Thankfully the Ferrari F2004, McLaren MP4-4 (which looked to be in full working order) Williams FW14 and others all looked the genuine article and one was actually leaking oil backing up the claims that most were actual working vehicles and not show cars. The F2004 and "MP4-21" we're certainly just rolling bodies.
http://img202.imageshack.us/i/dscf1029eb.jpg/ to begin with there are other images should you want to see the show MP4-21.