You can stuck in a trapdoor if:

1. Place any trapdoor next to any pressure plate.
2. Stand inbetween them.
3. Press "Shift"
4. While pressing it, go straight forward.
5. If you reproduce the trapdoor/pressure plate contraption use move through them without "Shift"
6. You move throught it back and forth.
   P. S. It doesn't work while not shifting, so you can't enter the trapdoor.