Didn't they accuse Candy Cummings of cheating when he unveiled the first curveball?
That's the legend. Some folks doubt he actually invented the curve ball, though.
My personal favorite example of the fine lines drawn in baseball is the stealing of signs. Legal on the field, but illegal if it uses any technology or personnel not on the field. Unless of course, the technology or personnel not on the field steals signs in one game, then uses the information "on the field" in another game. Starts to sound like an ad hoc pirate code after a while.