Loved playing this demo, despite its overall difficulty being a bit over of what I'm used to play. Levels are super creative and fun to play overall, though I found some little thingies worth mentioning.
> Layer 3 cutoff above.
> Pipe corner cutoff.
> Aeon already reported, but this pipe led me to a blank, black screen, and I died.
I'd put solid dirt up there to prevent "accidents" like this.
> More layer 3 cutoff, in the Star Warp submap.
> And that's just me nitpicking level design. Found this jump way too tough.
Died countless times trying it. But since it's a possible jump, it's okay I guess.
> Mario in the title screen, after the autoscroll stops, keeps moving right and this looks awkward. You could, instead of using Mario there and making the level scroll, you could use a standing still exanimated sprite that gives the illusion of Mario walking, plus the autoscroll effect for layer 2. More of a nitpick than an issue in fact.
> When I pause, music is silenced, as well as one of the SFX channels. And when I unpause, they don't return playing. Maybe take a look at the sound effect to see if there's something odd in it.
> Removing the "time is running out" SFX was kinda bad move, in my opinion, because due to exploring levels, I don't really pay attention to the status bar. I died various times due to time up so I'd just suggest to reactivate the time warning.
Anyway, I say again, your demo is very, very nice. The secrets and exploration are also very well implemented. I can't wait to play the final release. Keep it up!