Other than forgetting my keys once, and having to turn back (although it was a nice day so I didn't mind) I never forgot anything that would be a showstopper that'd ruin my day. I often forget this and that, but I accumulated spare clothing and accessories in the office so even if I forget something crucial like... pants, I have a backup Also, I carry stuff in panniers. One pannier is dedicated to tools, spares parts, tubes, extra hat, gloves, etc and I never take anything out of it so, it's always there.