Engineer can enter geometry while building.

[ghost]

The engineer is able to partially or fully trap himself within newly built geometry.

In this example, on a new map, I was able to completely enter the mesh with first person camera by building just three cubes while walking towards them:

Partially embedding the player character in the wall does not allow walking further into it, but the model/weapon nonetheless clips into it.

It looks like the game so far only relies on the weapon's push-back to prevent this.
It should instead check whether the player is standing where the new cube will appear in relation to the player, either pushing them further, refusing to build, or building it up as if someone was extruding the face (animated to provide a visual cue for the pushing effect, or simply stopping when the player is reached leaving behind a partial cube).