What I expected to happen was...:
A pressure plate would activate an iron trapdoor.

What actually happened was...:
After placing both the pressure plate and the trapdoor, the door duplicated itself after each use.
Steps to Reproduce:
1. Place an iron trapdoor and an adjacent pressure plate.
2. Step on the pressure plate.
3. Iron trapdoor opens and spews duplicates of itself over the area of 3 blocks.