When I pull the cooler from the car to drain the excess water, I really need to make sure I replace the drain cap. I thought one of the water bottles sprung a leak, when it was the cooler leaking, for two days. Block ice helps keep it cool, but not as cool when the cap is off.
So, double and triple check, in case I am sure I capped it, but it might have pulled open when scooting the full cooler back onto the seat and buckling it in.