Funny 3D Confusion
 

It happened, while I played with the particle-box on Paul Lutus' Web-Site:

https://arachnoid.com/particle_box/index.html

I rotated the box so that I looked from above, slightly from the right and a flat angle into the box. The particles where already rolling on the ground (reduce the energy restitution to get there quickly).

Then my thoughts drifted away just for a second and the Box was bent (into a truncated pyramid)! It took my a few moments to rearrange the image in my head. Now I believe I know how to reproduce the effect:

When the particles are all on the ground and you rotate the box slightly around all three axis, convince yourself that the particles are glued to a wall on the side, rather than to the bottom of the box. Edit: It helps to force the backward “wall” into the foreground.
ATTN: Doing the same while wearing my varifocals made me slightly giddy, so do not force too much.

It appears that the geometry must be adjusted in your acquired impression of the box, which means that the distortion, caused by the perspective, becomes a result of a different movement, than the one that you have actually performed.

Most funny is the fact that I can rotate the “bent” box again without losing the false impression.