Intonation.
Tune your open string, then fret it at the 12th. Because it's sharp, you move your saddle away from the pickups making a longer string length. Take a bit of tension off the string, move the saddle away a little, tune to pitch and try fretting the 12th again. You want it exactly in tune like it is open, if it's not, take the tension off the string again and move the the saddle some more until complete.
Grrr, the freak sneaked in and got there first.
