Many maps, such as Excavation Mutant Mode have a certain box(es) that allow players to enter them. If you were to enter the wrong box, you'd get the error "26_0" or such.
Safe boxes I've found so far are Crates in [Prison TD], [Excavation MM], [Cargo Ship ESCAPE], and [Egypt TD].
Although this takes practice, this is a simplified "tut" on how to get inside a box.
1)Stand on the edge of a box, facing with your CrossHairs on the corner.
2)ALWAYS holding shift, Hold Shift and Jump.
3)Still holding shift, you may crouch either when you land, or in mid-air.
4)Keeping your crosshairs on the corner, continuously move back and forth [W+S very fast], until you get inside a little bit.
5)Remember, you're still holding shift+crouch, and when you feel as if you've gone inside the box deep enough, let go of SHIFT and CTRL.
6)If you can see yourself inside, scooch to the boxes corner and hold control.
7)Have fun inside your box!