This level is slightly different depending on if you came from Stage 11A or Stage 11B. (Coming from Stage 11A, the level is the same whether or not you got the cutscene there.) The first part of the stage is mirrored if you come from Stage 11B -- i.e., you enter from the left instead of the right. Once you go through the door, the level is the same either way, with no mirroring.
At that point, you can go either left or right. After this, you will stay on that half of the level until the end; thus, there are two exits. This can affect the ending you get. Take the left path if you want to get the 'special' ending, but you will also need the cutscene in Stage 13. Stage 13 will also be mirrored depending on which exit you take.