Fortunately, this hasn't happened to me yet.
The first 3rd edition D&D game I ran was one where I told the players there were no racial choices, just take human during character creation and don't even bother writing it down since it's a moot point. The characters were in an oriental setting, so PC classes included samurai, shugenja, monk, and so on.
The first session intorduced them all to the honor system I was using (ripped from the Legend of the 5 Rings RPG) and a single combat encounter. There was a goblin ambushing travellers who stayed in a small shack on this frozen mountainside, the woman who owned the shack was also the goblin's wife. Anyone whose read the first story in Usagi Yojimbo may be familiar with this series of events.
The second session involved meeting giants from a nearby kingdom, now here's the trick that I introduced.
In the previous campaign, the PCs had all played humans travelling from a primitive culture and meeting very honorable and good-natured kobolds. The PCs, obviously, slaughtered any kobold they met, not trusting them at all since they are only kobolds.
I love kobolds, I always have, I have a special fondness for them in my heart. If they show up in my campaigns, they are usually good guys despite their low place on the food chain.
The honorable, good-natured samurai and shugenja that the PCs were playing were kobolds. I revealed this to them at about the same time that I described their previous characters. How their previous characters had reacted to kobolds was also how these "giants" reacted to the PCs' current party.
It was great fun!
The players got a thrill out of fighting their old characters while they simultaneously groaned that their current characters were now too short. The game didn't last much longer, and I hadn't intended it to, it was just neat to put the players into their former enemies shoes.