I haven't left caps unscrewed but I did something almost as bad, I fueled up prior to a flight and left the dipstick sitting on the pumps. I didn't realize something was wrong until I went to pre-flight for the return trip. As I was raised not to trust fuel gauges in an aircraft, I experienced a certain degree of trepidation, abated solely by the fact I recalled exactly how much fuel I had put into the aircraft so I knew I had put enough in to cover the entire flight regardless of what was left in the tanks.
Once I got back I surreptitiously wandered past the fuel pumps in order to return the dipstick without anyone noticing but someone had spotted it and handed it into the Clubrooms. After a sheepish walk back from the flightline, and a quiet word on airmanship by the duty instructor I had to buy a round later on in the bar. Lesson learned.