In this level, 3 things are irreversible - one so much so that the only way to redo it is to restart the entire game from the beginning!
1) The motion of a platform will be blocked by a non rewinding (green) platform if you reverse time too early in the level, which can only be undone by starting the level over.
2) The keys must be carefully used in the correct order; using the wrong key first on the set of 3 locked doors can again only be undone by starting the level over.
3) This level allows access to the world's painting, but attaching too many pieces together before using them to find the first constellation star will make this star impossible to reach (and thus the "true" ending unachievable), and this can only be reversed by starting the entire game over. Truly "Irreversible".