No, you're definitely not the worst mother! I think this happens to most people with their kids - when you put out a threat that you don't believe but the kids do!

When I was a kid all four of us were fighting in the back of the car on the way home from school- one of my brothers being the protagonist. Mum stopped the car and told him to get out. So he did and ran to the train station and proceeded to catch the oncoming train home by himself instead of get back in the car. I know mum was mortified and felt like a terrible parent when my brother assumed she meant it when she said to get out of the car.

All part of the learning process (for all of us!)