Well it's a collision file in the wrong place, there are like.. 54905495495495 different versions of SA with boxes in different places n such but if this is specific to you then it means your game's moved a model there or a collision's become corrupt and just put a new random wall up lol
If it's a problem and you really want it fixed, i suggest reinstalling SA and if that doesn't help then either get used to it or open the collision file for that area and look through them for any weird random looking spikes.