It's been years since I have forgotten anything major that lives in my pack. After each trip I clean everything, replaced used items, and put everything that don't need to stay lofted back in the pack. Lofted items after they are dry go into a huge cotton bag. So when I go I grab pack, grab the cotton bag, load a food container, and I am off without forgetting anything... except....
one trip I forget my hiking clothing.. which is a big problem since my hiking clothing is typically all my clothing except a vest, rain jacket, and extra pair of socks. Normally I wear "street clothing" to trailhead, change into hiking clothing, and then when I returned to the car change back into my street clothing. I left the bay area wearing trail runners (thank god), no socks, cotton shorts, cotton tee (it was quite warm). South Lake was going to be my trailhead. It was early summer (cool + bugs). Stopped in Bishop for lunch when I realized no hiking clothing. Thankful I had my debit card and Eastern Mountain Sport was a block away so was able to purchase a pair of socks, hiking pants, a supplex shirt, which were very much needed getting the bugs and sun on that trip.
Since then hiking clothing live in a stuff sack which is at the top of the pack until I hit trailhead so I don't forget them again.