The day I finished up my flight instructor practical test, I walked away with my fresh temporary certificate, and climbed into my rental airplane to fly home. I did a hasty walk around, as the airplane had only been parked for a short while, since the inspector and I returned from our flight. I obtained my clearance, taxied out, and departed.
I noted during the takeoff that the airplane felt ready to fly, and indeed took off just fine, but a glance at the airspeed indicator showed it reading zero. I looked out at the pitot tube on the wing to discover that it was covered by a tennis ball, used as a pitot tube cover.
I flew home, mentally kicking myself for that, and landed uneventfully.
Checklists are there to be used, and any time one walks away from an airplane, even for a short few minutes, one should preflight it again. I knew that. I knew better.
It only takes one small lapse to put one in a world of hurt. Most of the time we discover our mistakes early, and correct them. Sometimes we discover them a little too late. When we discover them late and move on, it's too easy to forget. When we discover them late and can't recover, we won't be around to remember.
Best to read about them, hear about them, learn about them, and use that to prevent those moments from happening to us. Even just a dream can be a wakeup call and a reminder; better to learn the lesson in your sleep than in the airplane, don't you think?