I got the skis, boots, and poles for 6 friends to go skiing. Except the boots didn’t make it into the car.
I was literally rubbing sticks together on one trip when I’d forgot the mini-Bic. I’ve packed an extra in my repair kit ever since.
In 1935 a Boeing prototype 299 (the future B-17) crashed because the crew had not disengaged a control lock. Even though the pilots and crew were all very experienced, airplanes had gotten so complex that the accident investigation concluded that they needed to consult a checklist.

When I was leading backpacking trips, we’d give all participants a checklist with required (tent, sleeping bag, etc) and optional (camera, notebooks, etc) gear detailed and then, upon meeting to drive to the Sierra, review those again. That’s even more important on a Scout trip, (unless you view forgetting your sleeping bag as a great learning experience, which it is).
REI, among others, has a reasonable list. There are LOTS of things you can leave behind on most trips, but reviewing the list lets you DECIDE to leave it behind.
https://www.rei.com/content/dam/documents/pdf/Gear%20Checklists/Printer%20Friendly/Backpacking_Checklist_PDF.pdf