If they would be impossible they won't be created.
Search up for speed runners, they know how to do this.
Just if you want to learn how to speed run, but it is to hard. Like they use bugs, and other stuff. But now more about your question.
They are just hard to beat and you have to think for a long time.
If they would be impossible they won't be created.
Here's Advanced Test Chamber 13, for example, in spoilers:
Best Answer
This does not appear entirely accurate, but it was the best I could find. Hope it helps! http://pastebin.com/hghRFv7Y