And then there are all the times I totally forget to pack something I need outside the house, but I can't exactly go back and get it because I'd miss the one bus an hour that runs to that part of town if I did...

Oh my, yes. This usually happens when I'm in a particular hurry, too (usually because I've spent half an hour looking for the second shoe, or similar). I started keeping my Oyster travelcard in my regular wallet just so I could be sure that if I successfully got on the bus, I'd reliably have my wallet on the other end! I try to make sure to return things I regularly need while out (including the wallet) to a bag I keep by the door, straight away after using them elsewhere. But, if I make a trip specifically to, say, return books to the library or to mail things, and at least one item is going to stay at home. I think it's some kind of law. :-|