Very Odd Thought.....

Yeah, because how could you prove if someone actually switched it..

I actually take pictures of (distinguishing marks are usually evident) my coins before I send them for TPG for insurance purposes.

That is just one more of the fears about coins and possible scams dealing with them. Almost every day as this hobby grows, someone will come up with a way and/or method on how to cheat and deceave us all. It would be possible for a TPGS to actually switch some coins and send back a counterfeit if they wanted to. For almost any coin in really high grades it would be difficult to notice a switch by most collectors. Think about it. You could be sitting on a pile of slabs of all counterfeited coins but since in a slab, no one would know. And if all the TPGS's got together and agreed to do this, all slabbed coins could be fakes. And if they really wanted to they could make their own machinery to make the counterfeits right there at their place. They would photo your coin, make a fake just like yours, send you the fake and you would never know.
Eventually all the valuable coins in the USA would all be in vaults owned by the TPGS's. And yes, they got paid to make the fakes by you, the collector, and you will never know for sure. Some day in the future a bunch of people that used to be the TPGS's will be all sitting in rooms full of our coins. And so many of us will all be sitting in rooms full of plastic slabs with no idea of what we no longer have.
The only solution I see to all this is to get a job at a TPGS and be one of those people I discribed above.
And note that all of the above is purely due to me not having anything better to do tonight.