Well, I could write a hundred blog entries under this title!
But today's lesson is: make sure you close the car door properly.
If you do not close the door properly, the internal light will be left on.
If you don't use the car for three days, this will run the battery flat. Dead flat.
If you are on your way to a meeting, this is not what you want to discover.
Then you will have to catch a cab to and from the meeting at a cost of $30, call your dad and admit your stupidity, spend $140 joining RACQ, wait for the nice RACQ man to jump start your car, drive around for an hour in a thunderstorm to recharge the battery, then spend $50 to get an auto electrician to properly charge the battery, plus spend $15 on a cab to get back to collect your car.
In summary, not closing the car door properly will cost you more than $200, waste hours of your time, and make you feel like an idiot.