You wanted a bug report, here's some 'light' reading for you:
Tube area still has 'suction' in ball form. Especially obvious when standing right next to the lower entrance at a standstill, and going into a roll. Sonic is slowly sucked in, and then half a second later, accelerates a little ways up before rolling back down.
Same issues as with Sonic. When gliding near to the ground near the lower right ramp, Knuckles will slide up along it.
Sonic clips through the blocks.
Same double suction issue as in the 1x S tube.
Spindashing up off the ramp on the right side of the screen puts Knuckles into the same vertical flipping animation as if he'd jumped off a spring.
I was able to get Knuckles to reverse direction inside of the tube for half a second before going back up in the right direction.
Now onto the truly crazy stuff. If you approach the S tube from the upper entrance and glide into it then immediately jump, there's a good chance you'll be able to break the constraint making Knuckles keep in ball form. Once you've done this, he'll flop around from surface to surface in his standing pose trying to walk or run on either of the tube walls he's closest to. Gravity is still in effect while he's in this state, but he cannot jump or roll into a ball unless you crouch again at the bottom (before exiting the tunnel) and charge up a spindash. Releasing a spindash will put him back into the correct sequence for it, e.g. always being in ball form in that area.
I had to screenshot this happening a few times as proof:
Sorry about the coloring on the top right screen, I accidentally hit Enter+PrtScrn just as I was doing the screencap, booting the demo out to the YOU CAN HELP TOO page.
Knux also can't get any distance while gliding if he's above the 'line' where the grass on the topmost hill would extend across the screen. You can test this by trying to glide after jumping on the right block moving up and down when it's near the top of the screen. Knuckles will float down vertically, and after crossing this invisible barrier will then start moving to the left or right depending on which direction he's facing.
2x Test Stage
I didn't find anything out of the oridinary in my tests with Sonic. With Knuckles it's a little different.
When you glide into the middle part of the half loop on the right side, he'll instantly transition from the gliding animation to the running one and continue to run up the loop and along the ceiling if you keep holding right. **Later note: after trying this on other areas in the demo with ramped areas, it looks like Knux will do this on every one of them. Was this part of the original game physics being recreated and not a bug after all?
This area was bug city compared to the other stages in the demo. The craziest bug I came across was one that caused Sonic to teleport through a floor onto the one above him. To do this I was trying to jump at the left pillar on the bottom floor in the center to make him clip through it to the left like he'd done a couple times already. This time he went straight up through the floor to appear on the ramp leading down.
This also made Sonic change layers (?) to the darker background ramp in the center area.
Various momentum based bugs for the area:
With Sonic standing at the left side of screen, run forward until you come to the first dip in the ground. At this area there are two flowers in the background texture. Just before Sonic hits the first one while he's running, jump. For me, this made him jump up, roll diagonally right down a surface that doesn't exist, and hit the ramp in a way that Sonic immediately transitions to the walking animation and walks up the right wall of the ramp. I hope that explanation made sense. This could just be a part of how the physics engine operates, but it looked incredibly odd to me.
If you stand right at the point of the dip I described and jump to the right, you'll see Sonic clip through the pillar and walk down the invisible diagonal ramp. I was able to repeat this easily.
3x res Super Climb w/Sonic
**After reading the thread, it looks like this issue's already been covered**
When Sonic is walking toward the right side of the screen and jumps toward the straight diagonal area, he will flip upside down and walk directly on that surface for a moment or so. This also happens on the diagonal on the lower left side of the middle block - on this one I was able to run left along the floor, jump up immediately beneath it, and make Sonic walk backward along the diagonal. Jumping from a standstill directly up to that surface also makes Sonic run partially upside down along it.
If you spindash into either of the ramp areas, Sonic will remain in ball form and spin in place along the corner area for a couple seconds before gravity kicks in again.
4x res Open Stage w/Sonic
On the swinging platform I ran into a couple curious things:
I had Sonic walk forward and roll into a ball when at the top of the platform's swing on the right side while he was walking left. As the platform swings back toward the left side and Sonic is moving in rolled up form, he'll temporarily get disconnected from the platform and roll in place in mid-air before falling back to the ground. I also tested this coming from the left side with the same effect. It almost looked like Sonic was running into an invisible wall that extended out from the blue orb the chain is swinging from. If Sonic isn't rolling far enough for his momentum to take him off the platform, this will not happen.
A variation of this effect happens if you move Sonic toward the edge without rolling into a ball. It will look like Sonic is walking on a flat horizontal plane for a moment before gravity takes hold and pulls him back to the platform again.
If you have Sonic in his arms flailing at the edge position on the left side of the platform as it's swinging to the left, it will dump him onto the ground when it reaches its highest point on the left side.
4x Multi Bridge Stage
More of a fun note than anything, but if you jump on the bridge and hit jump again just before it seems like Sonic is going to come into contact with it's bounding box, it will look like Sonic bounced off the bridge without it moving at all. If your timing is good enough you can repeat this indefinitely!
That's all the testing I'm doing for tonight. Another fine job LOst!