Knock on wood, I've never forgotten my PSK and other gear when going on a hike (I always plan these in advance - no spuir of the moment deals).

Sometimes, around town, I'll just be planning on going to the grocery store and end up doing hours and hours of something else on top of that (so far, always urban/suburban, not unexpected wilderness). I don't carry my PSK around town with me (maybe I should). I do always carry my Spyderco knife and my "special keyring" though. That keyring, funny enough, doesn't have any keys on it. But it has a small capsule with a spare $100 bill in it, another capsule with Advil and baby aspirin (in case of suspected heart attack), a pen, a whistle, and a AAA LED light. I always have my cellphone and wallet too. So I do have a minimum amount of stuff that may be useful. One thing I'm missing on my keyring is some way of making fire, but I'm not terribly worried about that in my urban/suburban messings around.

For urban/surburban I used to have an Altoids tin PSK that I carried in my pocket, but often times found that to be too large and it got left at home many times (OK, lots of times). Thus the "special keychain" that always fits in the pocket of what I'm wearing. If I'm PLANNING on an urban/suburban run-around, I have a small BOB that I carry with me. That carries lots of emergency stuff (PSK, first aid, food, water, firemaking gear, bandana, sunblock, sunglasses, etc.) along with entertainment stuff (a book, deck of cards, small pieces of rope to practice knot tying, an iPod, etc.)