Mickey, the slip-up isn't whether he is operating on a real tiger or not, it is the fact that the tiger's "skin" is rubber. I'm not a biologist or zooliogist, but I'm pretty sure that a tiger's skin isn't made of rubber.
Well, its obvious that a tiger's skin isn't made out of rubber but they needed to USE the rubber to ACT like skin instead of using a REAL tiger. There's not much you can do to act like the skin is real.
Everyone knows that they were not going to operate on a real tiger. Everyone also knows that a tiger's skin is not made of rubber. The point here is: The rubber LOOKED like rubber. They could have done a better job at disguising it to look more like real skin.