When Middle Son was getting dressed for school this morning, he put his shoes on the wrong feet. And he left them that way. He appeared to be perfectly comfortable.
I saw that and remembered that Oldest Son used to do that as well. From about ages 3 to 5 (I don't recall exactly, but I think it was something like that), he insisted on wearing his shoes on the wrong feet.
Middle Son craves proprioceptive stimulation; he wants pressure on his body. He likes to be squeezed and mushed. He likes to jump off of things and feel the impact. It's a common craving for people with autism. One of Middle's occupational therapists said that their nervous systems are under-sensitive to proprioception, so then seek out additional stimulation.
When Oldest Son was wearing he shoes on the wrong feet we speculated that it was driven by a desire for pressure. Middle doesn't do it often. Most of the time he gets his shoes on the correct foot. But if he doesn't, he either doesn't notice or doesn't care. I don't know which.