you CANT make one pixel wall
Interestingly, real world electrons have a similar effect known as quantum tunneling, which a electron with higher energy has higher chance to pass through a "voltage wall".
Because of how computer simulation works. Fast particles move across many px per frame so it would slow down tpt severely if it checks every single px it passed. So tpt just skipped checking some px. Faster particles has higher chance of skipping walls. Fortunately, for most particles, it is guaranteed that 2 px thickness wall must be detected. Note that particles like dest, energy particles, etc do not provide this guarantee.
I know, this is annoying. I still don't understand why one pixel thick walls cannot stop fast moving particles.