Not my biggest lie, but the one I believed the longest.
For context I'm in America.
If you turn the overhead light on in the car while driving at night, the cops will give you a ticket (a fine).
I was told that as a child whenever I would turn the light on in anyones car at night.
Bonus lie that I believed about just as long:
"If you dont stop talking you will scare the fish away"
I was told this whenever I went fishing, and I didnt like sitting in complete silence due to my trauma.