You can make the crate invunerable by setting its strength properties to 0 (zero) in editor (right click entity).
If you push a box into a corner obviously you have to get behind or to the side of it to push it out - alternatively you can pick the box up in game (right click) and move it if Physics are set to on and the entity is dynamic.
If the crate is light enough then it may bounce out of a corner if you push it hard enough. Normally a realistic crate weight may prevent this just as in real life. If you get it jammed in a corner position then you would not be able to move it easily.
Your cate should not actually get stuck in any position by means of its polygons getting stuck in a wall though this could happen I guess. Some things can get caught up in FPSC though I have never had a crate or box do this myself. Enemies and occasionally the player does, collision for character entities, enemies in particular deteriorated between the EA and V1 versions as the AI avoidance routines if theres such a thing are somewhat less stable and leave somethimg to be desired.
