If you place two blocks and then trapdoor on bottom of them both you can go trough them if u step where the bottom one is, make it horizontal and then make the upper one horizontal u are still standing and u are in them both. If u have the bottom one vertical and make it horizontal and then make the bottom one horizontal as well u can crawl even if u brake the upper one. If you stand where the trapdoor is and then make it horizontal u can't crouch on it.