A couple of favourites from my time working in a music shop:
People who pronounce "Les Paul" as "Lay Paul" like it's French.
The person who referred to a Gibson Flying V as a "Flying Five" as he thought the "V" was a Roman numeral, despite the body shape providing irrefutable evidence to the contrary.
(I have to say that there are far more words used by musicians that piss me off than mistakes made by non-musos. Things like "Mojo", "Vibe", "Groove" & "In the pocket" really make me cringe).