This patch fixes a bug in smw that sprites that fall off screen still interact with objects. This can be proven by koopa shells suddenly veer left or right when they pass through the solid blocks as well as triggering the custom blocks when they shouldn't.
So this stuff happens with a clean rom that is then patched with SA-1 and then this patch. Tested before this was patched with only SA-1 inserted and everything was normal, then I patched this in and we get blue shell-less Koopas who walk on air and Rex who defy logic.
I really do not understand how these easy to identify problems are missed by the patch moderators. Literally the first level for crying out loud (if you go for the Switch Palaces).