To the best of my knowledge it's been tried twice, detected both times (the balls are still damn cold and the umpires get to see them more than an hour before game time) and never actually happened in a Major League Game. I don't doubt that someone has pulled it off at a lower level, but its more a theory than a regular occurance.

That would very hard to do but anything is possible in this world.

I've never heard of this one, but I can see how it would be effective ...

It's very illegal but I love this one. . .if your team can't hit a lick and you have heavy hitters coming to town, put the baseballs in the freezer for 24 hours! Take them out several hours before the game, wipe off the condensation and bingo! It feels like you're hitting a solid rock. . .the balls will barely leave the infield.