One thing I forgot recently that I wished I had was one of those tiny fold up disposable raincoats for my day pack. Rain was not predicted that day yet it rained while I was a considerable jog from the hotel.
Also, my compact but very servicable first aid kit. I had an opportunity to treat someone in need but had nothing with me. And you know the rules, if you don't have universal precautions ie: nitrile gloves, CPR mask etc you don't touch.
I had the skills but no kit so I did the next best thing, called 999. Next I begged the store close by to give me their kit. The first place said they didn't have one. I burned up about 5 minutes searching for one. At minimum I always carry gloves, ASA and face shield mask on a keychain...but not that day. Kind of ruined my morning b/c as EMS I know better. First Responders will understand what I'm talking about...