The bug

With a certain setup, it is relatively easy to phase through a wall block and end up inside of it.

To reproduce (screenshots attached)

1. Create a new world in creative mode.
2. Open the creative menu, and grab a wall block, as well as any solid block.
3. Place two walls adjacent to each other.
4. Stand on the short side (doesn't matter which), right up against the wall.
5. Jump and place the solid block beneath you.
6. Walk backwards.
7. You are now inside of the wall block.

Expected behavior

You should not be able to put your player inside of a wall block, or phase through it in any manner.