How my own idiocy turns around to save me:

Me: stuffing big hardcover book into my smaller bag (I really didn’t want to carry a bigger bag), being surprised that it actually fits, not realising that it only fits because my wallet is not there.
Me: hour later on the train, finally realising why the book was able to fit. Realising that no wallet means no coffee money and no lunch money. Trying to think if 900 yen ($9) I probably have on my train card will be enough to by something… somewhere.
Me: 5 mins later, sticking my hand in my pocket and finding my credit card, which I forgot there when I went out to convenience store 3 days ago.

I usually never carry cards in my pockets.
And now my one slip-up saves me from my another slip-up.