I play Action X from my hand.
Once Action X is finished resolving, the gamestate is exactly the same—it is as if Action X had been an Action card that said "do nothing". All cards are in the same places they were when I put Action X in my play area, I have no more (or fewer) coins, actions, or buys, etc.
But! Action X didn't actually do nothing! During the course of resolving Action X, a single card ran around the mulberry bush, passing through four distinct locations and stopping in the fourth, right back where it started.
How did this happen?