With TT bikes I think it happens all of the time..... I almost bought a used TT bike off of Ebay that had belonged at one time to an American pro woman. It was rebranded as a Lemond, but the stickers were peeling off and you could see that underneath it was something else entirely....
Sometimes though they force the guys to use their bikes... I heard a story once, I think it was Magnus Backstead? One of the big guys tall guys - he wasn't allowed to get a TT bike from another company and their sponsor didn't make a bike big enough for him.... and they wondered why his TT results were not better...