Ok, I always see people wondering how to do this, and I thought that I might as well fully explain completely in-depth how to walk thru walls in SMB1.

1. 3 block high platforms Walking thru platforms 3 blocks tall or higher, is extremely easy. This method is possible with power-up states 1, 2, and 3, and can be done holding B, or not holding B. Simply jump into the top-right block, and start holding left once you are about half-way thru the block. Then you can start running.

2. 2 block high platforms This one is much harder. It can be done in power-up states 1, 2, and 3. See the end of 1-2 in the 2 latest WR speedruns, by Kosmicd12 and Somewes for an example. (1-2G). This can not only be preformed in 1-2, but in any level with a pipe or platform that is 2 blocks high. (Not counting the ground after a hole, that just wont work unless in power-up states 2 or 3 after stomping an enemy that isn't a goomba, so I will not explain that in-depth). Before jumping into the pipe, make sure these two variables are true: you are NOT pressing B (Easy ways to de-accelerate are to press R + L, just waiting a few seconds, or jumping, and letting go of B midair), one block away from the pipe (16 pixels), and only jump for a frame. Then, similar to the first one, push left half-way into the block, and start running. (I will finish this later cuz i'm lazy)