We have nothing leftover, because we budget every single dollar that we make, and every single dollar that we spend.
Sometimes we'll have a small surplus, because the gas bill was less than expected, or maybe we didn't eat out as much as we budgeted. We then put that surplus into one of our savings accounts, that are all earmarked for various purposes, like new car, vacation, etc.
Consequently, unexpected monthly expenses that arise after the month's budget has been written come out of one of those savings accts. There's one for gifts, one for clothes shopping, and emergency fund for emergency dental, medical or vet visits.