If you manage to get your head inside of a solid full block (stone, glowstone, leaves, etc.) by using riding a minecart or using the setblock command, you will see the texture of the block under your feet (or under the minecart. Also happens when riding horses if you get the horse to stand inside a block).

Also, if you place a block, say one to the north of the one under your feet, and sneak north a bit (so your hitbox moves onto that block, but your head is still in the solid block), sometimes the texture you see will switch to that one. This only happens sometimes, and only happens to the north, south, east, and west, but not diagonally (as far as I've tested).

To reproduce:

• Place a noticeable block under your feet. Sponge is pretty obvious.
• Place a solid block inside your head (stone, glowstone, etc.). The easiest way is probably by using /setblock ~ ~+1 ~ minecraft:glowstone or something similar.
  -You will see the sponge texture as if your head was in a solid sponge block. If you put glowstone, redstone block, glass, etc. under feet, you will get a transparent x-ray texture, even if your head is actually inside of a solid block like stone.