This method has been used already & is not reported yet, so i guess i'll report it...
As our good friend Darwoth pointed out yesterday (he really didn't want me to leave during our candlelight dinner)
If you build Mine Supports around the ladder, you are unable to exit the mine.
If this happens on a personal claim (not owned by yourself), the person entering the mine will KO & be trapped forever (& possibly getting killed)
The supports are undestructable & you can't walk through them, leading to gamebreaking mechanics !!