I'm having a problem with my guitar and I have no idea how to fix it.

I was playing a riff where i had to play the 12th fret of the high E string and when I played that I noticed it was off-note (or false), but when I played that same string open, it sounded right. So I thought that the fret is wrong, but the weird thing is, that it's just off-note at the high E string and not at the other strings.

So if anyone could help me with that it would be wonderful, because it is really annoying to play with false notes.

Thank you!
intonation (sp?), i didn't even have to read the whole post and i knew this answer. it's an easy fix, don't sweat it.
Cream fan club member #11.