One bit of trivia I learned the hard way. When working for a long while with the driver's door open, it is not enough to switch off the courtesy lamp on the roof. The delay relay heater continues to draw current as long as the door remains open.

No real consequence from the dead battery, except losing all the settings from the customer's fancy sound system. Newer door switches have a twist-to-disable feature that can be hard to set through the rubber boot.
--
Art Benstein near Baltimore

Experience is something you don't get until just after you need it.