One of the things that kids do in metal shop is open up a fifty cent piece by using a lathe and going just inside the reading and hollowing it out, they then take a dime and grind it smoth on one side to fit in the hallow spot. Then taking another fifty cent piece on the lathe you cut the reading off and grind it too. You put the dime inside, put the new back on the fifty cent piece and seal it. If it is done right, it is very dificult to detect unless you rattle it and the dime slops around a bit. I have one somewhere and will try to find it and post it.
Show on the US Mint on travel chanel couple of nites ago show a machinest hollowing out a coin and putting the face of a quarter in its place. Took a lot of precision to do and the right equipment but in the end it looked like a real error coin.