In what appears to be a rare case, collision seems to be able to turn on for two players. I don't have any visual evidence of the matter, but it just happened to me during a run on surf_how2surf. I had just teleported to stage 12, and I seemed to do it at the same time as another player, and we got "stuck" in each other. I say that with quotations because we were still able to move independently, although only to a certain extent, and it became almost impossible to do so until we teleported back to the beginning of the stage. This teleport then appeared to render us "unstuck". This could just be a spinoff of the old camera bug where the camera has a hard time following the player through teleports at times, but through all the times I encountered that I never had a harder time moving.
Unfortunately it's likely there isn't much we can do about this one. Typically mappers when they want to slow players down when changing stages will attempt to change the players movement type amongst other things which may cause such an effect. I'll leave this one open and if it happens again, please update me.