At the bottom of a car glove compartment, along with a few fuzzy kleenex and half used rolls of breath mints, there are typically a few documents: insurance papers, a copy of the transfer of ownership form, emissions testing certification, and the car owner's manual. If you're stopped by the police, you'd better have the insurance papers on board, and be able to produce your driver's licence. Airplanes have similar requirements.
You rarely get pulled over by a police helicopter during cruise, but after landing it's quite possible that you'll be approached by two guys (they always travel in twos) with Transport Canada jackets and little dangling ID badges -- they stand out pretty clearly at small airports where no one else wears ID. They ask to see your documentation and sniff around trying to find or make you admit bad things about your aircraft and your operation. And they will ask to see the airplane documentation.
For an airplane you need to have on board proof of insurance and the pilots' licences. The owner's manual is actually required to be on board, as is a certificate of registration, proving someone owns the airplane, a certificate of airworthiness, proving that the airplane was once considered safe to fly, and a weight and balance document declaring how much the airplane weighs with all its installed equipment. There's also a big book called the journey log that lists everywhere the airplane has been, when it took off, when it landed, who was flying it and what went wrong with it. Some operations also require a radio licence.
At my company, the essential documents are kept in a little pouch that the pilot picks up at the dispatch counter with his or her flight assignments. The journey log is too big to fit in the pouch, so it's separate. Before flying, one checks in the journey log to ensure that nothing critical went wrong with the airplane and hasn't been fixed yet. It's important to rememner to bring the documents with you, after stopping to check them. Combined with the title of this post, you can see where I'm going with this.
Yes, we left the documents on the table in the dispatch room and did a flight without them. Yes, we noticed the absence of the documents when we tried to fill in the journey log after a series of flights. Yes, we snuck sheepishly back into dispatch. The documents were not in evidence, and just seconds before I opened my mouth to ask if anyone had seen them, I saw one of the more zealous Transport Canada officials, ID badges dangling from his belt. I walked past him to the dispatch counter.
Fortunately, he wasn't there because of my transgression, but had just dropped in for free coffee or something, and I got away with my oops. The dispatcher had safely set the documents aside. Being nice to dispatchers pays off. A dispatcher who didn't like me could have got me in a lot of trouble over that. I snuck off and filled in the journey log without anyone else being the wiser.