That's what the tiny inside pocket of your jeans are for: folded up tenner for emergencies or leave a well hidden one in an envelope somewhere in your car.
I used to do that instead of carrying a wallet/cash card when I got paid cash for travel expenses.
i tend to leave either a fiver or tenna hidden in my car for those times i need it in a emergency its always best to just in case of that reason of forgetting bank card