My dad was really interested in fonts and typefaces. He could tell you when a particular font was invented, what it's original purpose was and why you should or shouldn't use it now. He also spent some time creating his own font, although I'm not sure he ever got it quite right.
Dad also had a huge knowledge of military planes (he'd been a pilot in the army) and his most annoying habit was pointing out that the engine noise of a plane on tv or in a movie was wrong, and which plane it was actually from.
My DH has a similarly annoying habit, except with him it's guns and other weapons. He's constantly telling me how the gun being used by the hero in a movie can't hold that many rounds or some other factoid I really don't care about. He's not even that into guns, it's just come from years of gaming.