When I played trumpet in high school I had to read music, but my teacher used a weird method where we called the notes by a number, rather than the letter of the note. For example, the middle line of the treble staff is a "B," but we learned it as a "7." So I play trumpet by numbers, but if I want to play something on a guitar or piano I cannot do it by sight because I still read the notes on a staff as numbers instead of the proper letter. I have to write the notes out on a separate paper transcribing the numbers to letters. It's a pain to theoretically be able to read music, yet not be able to read it at the same time if that makes sense.