I suppose it's already been mentioned about Lady Opera...
Beating 5-4 gives you level select, which lets you into 6, 7 and 8, which I assume is unintended in this hack.
That creates some interesting situations. Hit the vine block (=end-of-level block) in 6-2 and get stuck.

7-1= original, 7-2 = 2-2, 7-3= 2-3, 7-4 = original
8-1,8-2 = original, 8-3=1-3, 8-4=3-4?
Beating worlds 6, 7 or 8 just stops and waits for you to press B to go back to the title.