I remember the day I knew something

Nobody’s perfect and if you knew me, I think you’d agree that I’m one of the most non-perfect people you could meet. You might know something about a particular topic that I wouldn’t have a clue about and I might know something that you might not know about. In truth, I know very little. Because of this, if the rare occasion arises when I do actually know something, I like to let people know that I know about it. This makes me an annoying son of a mocker. Recently we were filling the car up and one such occasion raised its head in the form of the below sign which I just had to correct: