It's not a game-breaker, but all it is is that when in Adventure Mode, you can punch an Iron Trapdoor, seemingly until it breaks, but when it reaches breaking point it will just go back to normal. Also if you stopped breaking it, the cracks stayed there, which is actually pretty cool, so maybe a new feature (hint hint?)
Environment
x64 OS, Java 8? Pretty sure it's 8, but Windows 10 isn't letting me check it :\
Is this still an issue in 15w44b? If so, please explain the issue in detail and provide steps to reproduce it.