If you place a trapdoor (just make sure it is on the top side of the block) and try placing bells, lanterns (same with the trapdoor on the bottom side of a block), candles, amethyst clusters, and flower pots, it will not let you do it even when you are crouching (and also try placing bells and lanterns under the trapdoor).
And if you place a trapdoor on the ground, open it, and try placing a tripwire hook on the other side of the trapdoor it will not let you do it even when you're crouching. All of this blocks can be placed on trapdoors on Minecraft Java Edition.