I remember, when I was a kid, that I had a toy piano where the notes C, D, E, etc. were labeled 1, 2, 3, etc.

Later, I tried to get a computer to play a simple melody and assumed that 1=A, 2=B, etc. The result sounded wrong, and eventually by trial and error I figured out that if I put C# in place of C, then everything sounded good.

I asked my father why, and he gave me a version of "you'll understand when you're older". My father was a wonderful man, but this is still one of my sadder memories from childhood.