I've got three "duh" moments in my family:
Ages ago, I took a July trip to Florida with the parental units to visit the granparents. We took my niece along to take her to Disney World and meet "Noky-no" (pinochio) - she was about 3 or 4 at the time. My dad was keen into photography at the time was eager to get lots of photos of her there. It was a day of non-stop misery 'cause this kid was frightened of every ride, of all the characters, and in general was very trying. But we did get some good shots with the camera. And then we get back to the hotel, and my dad realized during the entire time he had failed to put film in the camera! The next day, on the way home, we stopped at DW again just so he could get pictures of her there.
Second story - I was traveling with my aunt and uncle. We had stopped at a rest stop, where they switched sides so she could drive. She complained that she couldn't get the car to work. She forgot to start it!
Third - Another, recent trip with my parents to meet someone. Afterwards it was decided I would drive back. I get behind the wheel and try to start the car - it won't start. I keep trying, until my father informs me the car was already running ๐ณ (they have a luxury car - from inside you cannot hear or feel the engine running)