If you just use a plane, the rays will be bent once upon entering the glass layer.
In a normal glass pane (with thickness) the rays are bent upon entering and then 'unbent' (bent in the other direction) when leaving the glass.
You can see what i mean in thiis google image: http://www.infoplease.com/images/ESCI114REFRAC001.gif
This means that in the first case the rays are traveling in the bent direction far longer & never 'unbent' which means you get a distorted image.
I always use glass with thickness for better realism. You can use glass as a plane, but then you need to set the ior to 1, which means the rays don't get bent.