Question about a S3k's collision bug

Hello.

It has come to my knowledge, many years ago, that S3k engine has a bug in collision detection. If I remember correctly, said bug had something to do with Sonic getting through chunks and/or doing dead-stops at slopes?
All caused because collision code checks for a wrong bit... ?

What's the nature of this bug? Is it indeed a wrong bit getting checked? If so, what's the mistake in itself?

 

I swear I've seen something like this in both Sonic 1 and 2. In the Sonic 2 GitHub disassembly, it's here:

Code:

ObjCheckLeftWallDist:
    add.w    x_pos(a0),d3
    move.w    y_pos(a0),d2
    ; Engine bug: colliding with left walls is erratic with this function.
    ; The cause is this: a missing instruction to flip collision on the found
    ; 16x16 block; this one:
    ;eori.w    #$F,d3
    lea    (Primary_Angle).w,a4
    move.b    #0,(a4)
    movea.w    #-$10,a3
    move.w    #$400,d6
    moveq    #$D,d5
    bsr.w    FindWall
    move.b    (Primary_Angle).w,d3
    btst    #0,d3
    beq.s    +
    move.b    #$40,d3
+
    rts

Since it's in both Sonic 1 and 2, its' possible it slipped into S3K. Of course, I could also be completely wrong; I'm going out on a limb here. Hope it helps...

 

That was fixed in S3K:

Code:

ObjCheckLeftWallDist:
       add.w    $10(a0),d3
       eori.w    #$F,d3    ; this was not here in S1/S2, resulting in a bug
; sub_FE44:
ObjCheckLeftWallDist_Part2:
       move.w    $14(a0),d2
       lea    (Primary_Angle).w,a4
       move.b    #0,(a4)
       movea.w    #-$10,a3
       move.w    #$400,d6
       moveq    #$D,d5
       bsr.w    FindWall
       move.b    (Primary_Angle).w,d3
       btst    #0,d3
       beq.s    locret_FE6C
       move.b    #$40,d3

locret_FE6C:
       rts
; End of function ObjCheckLeftWallDist_Part2

The whole 'stopping dead at a slope' thing just sounds like Sonic going so fast that the game fetches a collision height too far into the block, which winds up being too steep compared to the last height fetched, causing the game to assume it's a wall.

The only 'checking the wrong bit' bug I know of is debatably not even a bug. It's the one where, in order for Sonic to be able to run up a wall, its blocks need to be solid on the top as well as the side, since it checks for that bit exclusively.