Here's an interesting bug I've found. Normally getting a feather causes about 30-40 frames of delay while transforming, while the game plays that sound effect that sounds roughly like seven cats fighting each other to the bitter death -
reereeREEROEOAORAROAOAAA - back and forth in the left and right speakers. Well, I was playing Cheese Bridge from SMW and collected a feather above the screen and this delay was skipped (the sound effect remained, however). Here's a
demo of the trick in Vanilla Secret 1 of SMW.
So on Misty Star World, you need to take the fire flower at least once in the level, and then take the feather back because flying is fast. Since it's an autoscroller, I had plenty of time to set up something, and
this was the result - the game froze. Not completely, as I can still pause, and I would assume I could start-select out of the level had I already completed it, but frozen enough.
Why? I have no idea.