i just changed my strings for the first time in about 8 months (really long time. i know) and the highest string is not intonating properly. at the 12th fret the string is flat but at the 24th the intonation is perfect. this only happens on the first string and ive never had this before. anyone know how to fix this?
It also actually could be the string. You did not mention if the string was a nylon or a wound string, but you probably mean a plain steel string (it seems like it's a solid electric). You also did not say if the replacement strings are the same type and gauge.
Anyway, a string can be defective by having inconsistencies at various points along its length. Why don't you first try changing that string to see what happens (if that don't work, try changing whole set)
There are many causes to poor intonation
A bent neck.
A twisted neck.
The guitar saddle can be on a bad angle or height for the specific string. The most common type of guitar for this is a Stratocaster since the there is a mini saddle for each individual string.
Dints between each frett from old age. Concaves on the fretboard is a common cause for old aged guitars to have bad intonation. Since there is a dint on the fretboard from wear and tear, when you play on the dinted fretboard, the string will sink into the dint area beyond where it should and therefore causing more tension on the string which causes it to fall out of tune. I have this on a vintage strat I own at home. It looks like this.
Hope this helps. :)