Found a bug! (My GF did, actually)
SPOILER
When you arrange the color cubes correctly on the outside, to open de apse... If you put first Yellow and Blue, then Red, and BEFORE putting the Green one you hit the Red cube with your body (trying to reach the green placeholder, for example) you can make it go outside and fall, but after you put the Green Cube, the Apse will open anyways, with the Red Cube outside it, being able to take it to the Apse without needing to block the entrance with the plank.
/SPOILER