Douglas Adams said this:
Quote:
"There is a theory which states that if ever for any reason anyone discovers what exactly the Universe is for and why it is here it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another that states that this has already happened."
Make a map that embodies that idea. for example, if you solve something in too ninja a fashion, you get thrown back, and the whole puzzle morphs into something harder, right before your eyes. you hear motors whirring, walls becoming non-portable, huge goo pits opening up. fires starting, etc.. glados chides you for thinking you're so special. this doesn't have to be just a single iteration event.
it wouldn't be outlandish to have the gun thrown from your hands and you have to get it... somehow...
but use your creativity. you could do any or none of the things, but just look to the Adams quote for guidance.

