Certain block placement allows character to walk through glass

In my world I have a gold farm with a kill chamber. The front of the chamber consists of a smooth stone block with an oak trapdoor on top. On the square above this is a clear glass block.

 

I discovered last night that I'm actually able to jump on top of the trapdoor when it is in it's open/horizontal position and walk through the glass block into the chamber itself. I am then able to walk back out through the glass again.