Since there seems to be renewed interest in this game, I thought it'd be meaningful to make a post about this glitch. Basically it's possible to clip through the floor (in particular, at the intersection of two lines). I'm not quite sure how this works, but I imagine a number of movement variables come into play:

Fortunately in this stage, there's actually floor immediately below the corner you can clip through.

This is the kind of glitch I was trying to reproduce! I seen it with enemies before but never done it with the player sprite! Thank you, I still have some logic to work through on this but this can be frame analyzed by almost anyone to see if they can reproduce this, or find out what determines if the clip happens or not.

Yeah, I always imagined this trick was subpixel dependent upon running into the corner, but Hourglass is so unstable for me with this game I never had the chance to dig deeper. I couldn't find a consistent setup for this which is why I omitted it from my any% runs, but if anyone can find some clues here, this could open up a lot of applications across all categories.

I don't know your Hourglass setup, but perhaps using what is called a virtual machine with Windows XP installed inside would help. VMWare Workstation, VirtualBox, both are excellent ways to run Windows XP inside its own virtual environment, and I remember reading up on Hourglass working best on WinXP.

I also have a Pentium 4 ~2.5GHz desktop lying around not being used for anything, which is enough to run NSML according to its system requirements. I'll set up the P4 with Windows XP and install Hourglass, and see if I can learn how to use Hourglass when I have the free time.

I think I might have cracked the code. https://drive.google.com/open?id=11Rxqrcv7mHXEnU32_axJHrqQ6R7irFDJ

I've gotten a pretty good success rate getting the clip by sliding off of this slope, and if you screw up, you don't lose too much time killing fairies and you can take a few tries. It probably won't be worth it to get it 2nd or 3rd try, but I haven't really done the timing on that yet.

Regarding the setup, from what I can tell you have to start just below the first corner, hold down, then jump right before the second to last corner and immediately hold right (and you should be holding the run button the whole time). It seems like there's a decently large range of where you can start sliding; presumably you'd compensate by jumping at a slightly different time, but it doesn't seem pixel perfect.

It doesn't seem to work while big, which is inconvenient for 100%, but a small reroute will let me work it in to the route without much change to item management. I don't think I'll route it in yet, considering after I get it I'd have the autoscroller, then all of the tight tricks in Alice's stage, then 2-1, but this definitely seems like a viable setup.

After doing some timing, I haven't broken 3 seconds of timesave on my setup over not clipping, and usually I take some of that trying to position myself better. On top of that, I lose time picking up a powerup in 1-5 (it's an autoscroller, but you do get the short pause for picking up a powerup). This definitely isn't worth doing for 100%. It might be worth doing in any%, but it might be better to use bjw's faster but less consistent setup.