This may be common knowledge to some, but I really had no idea the Google AdWords Professional exam was timed. You have two hours to complete about one hundred questions. You cannot pause the test or stop it and pick it up later.
Personally, I am an extremely fast test taker. I was the one in class that always finished first. Didn't mean I got all the questions right, but I was fast. But there are many people out there that are extremely slow at test taking. Do you think two hours for 100 questions is enough time?
A Google AdWords Help thread has AdWordsPro, an official Google representative, saying that he/she wanted to pause the test him/herself but was unable to. Let me quote:
When I took it again recently I, too, wished I could have paused it for a quick trip to the restroom. Too much coffee during the final pre-test studying!
Forum discussion at Google AdWords Help.