I figured out how to do the bug consistently. On level 2:
1.Grab checkpoint (by this point, the green box around the red button is non-collide too)
2.Reload, hold right and jump, ONLY ONCE, at the edge, and you fall through the area where the red button is. (dont let go of right at all thru this)
I can give it in a bit more detail, maybe a video too if you need it.
Edit: this works on any level, as long as you have a checkpoint (not sure if it only works on first) and jump and hit the white ground, without hitting anything else.