For me it will be locking myself out of my car. In the past, I used to lock my car without the remote control by manually locking it before closing the car's door.
One day, I was kinda in a hurry, so I didn't know that my car's key was still inside the car when I had locked the door. By the time I realized it, it was already too late.
I had to miss my appointment by explaining to my client before rushing back home to get my spare key and returning to drive my car.
This time, I rely on the remote control so that I have to have my key in order to lock the car.