Anvil/Chest Manipulation

Anvils don't have a fixed collision box. Instead there is exactly one universal collision box for all variants (North, East, South, West), which is updated whenever the player looks at one. Normal anvils, slightly damaged anvils, and very damaged anvils are not independent.

Chests behave in the same way, although only Double Chests have a different collision box. Chests and Trapped Chests are independent.

By standing next to an anvil/chest, and by looking at another variant, the player can update the collision box in such a way that there are now considered "inside the block". The same trick can be used to artificially extend the length of the anvil/chest, and manage to stand one or two pixels further than intended.

You must be careful not to look back at the original block, as that will reset the collision box to normal.

Comparison of collision boxes

Anvils are a simple 1×0.75 bounding box (16×12 pixels), that can be oriented along the X or Z axis.

Single chests are a simple 0.875×0.875 bounding box (14×14 pixels). The "double chest" variant extends one direction by 1 pixel.

Limitations

On Singleplayer, the player can usually walk through blocks once they clip into one (in this case, the manipulated anvil/chest).

On Multiplayer, the server will typically lagback the player when moving inside a block, unless they are less than 0.0625b away from the edge.

In 1.9, this mechanic was patched (each variant now has its own collision box).