Knuckles can climb walls with the Not Climbable flag checked while in 2D

[742mph]

Pretty self-explanatory. I don't really know much about coding, but I would guess that what's causing this issue is that the game thinks of 3D climbing mode and 2D climbing mode seperately, and it checks whether a wall is climbable when deciding whether to go into 3D climbing but not when deciding whether to go into 2D climbing. That's just a theory, though.

This image is from an unreleased, half-finished attempt at creating the steampunk Metal Sonic battle in Whackjood's level collab's Steam Engine Zone Act 3. Knuckles is climbing the side of an invisible but tangible FOF that borders the arena. The target sector's walls, however, all have the Not Climbable flag checked. Though I don't have any visual evidence, I have also run into situations where this has occurred with a normal sector and not an FOF.

 

[SSNTails]

In 3D, the climbing involves the linedef, whereas in 2D, it typically only utilizes the neighboring sector. That's because no wall sliding occurs in 2D.

 

[Torgo]

As you might be able to see in the srb1 remake levels, I went around this by using walls near the top of the level so that knuckles wouldn't be able to climb to the very top since there was a physical wall above him. I only did this in the knuckles only levels (I probably should have done it for coop too, but it is only mostly cosmetic). In the end, I think it was better then having walls he suddenly can't climb for inexplicable reasons.