Found another one. Usually I do this: Unlock with remote, get in and lock all doors with the lock button on the drivers side and then start the car.

BUT.... if you do this: Unlock with remote, get in and lock all doors with the lock button BEFORE your passenger manages to close the door. The moment the last door closes after the lock button is pressed, the alarm system is armed! If you didn't notice the red light and proceed to start the car, guess what happens?