In most worlds, the oldest state just gets thrown out; you keep playing as usual, it's just that if you were then to try and rewind all the way to the beginning

In most worlds, the oldest state just gets thrown out; you keep playing as usual, it's just that if you were then to try and rewind all the way to the beginning, there would be a limit on how far you can go. In world 4, though, where you can walk to the beginning of time just by walking to the leftmost part of the level, I actually kick the player out of the level when there's no more memory. I have never heard of anyone noticing this.