A bad guitar string can cause buzzing on just one fret
I had this happen on my guitar. I'll tell you what happened, how it happened, and how I determined it was the string that was the problem.
My strings were getting old on my Schecter, so I installed a new set. After doing that, I wanted to make a small bridge height adjustment.
There are two ways of adjusting bridge height on the Schecter I have. Method 1 is to loosen the strings and use a flathead screwdriver from the top to adjust the bridge's set screw height. Method 2 is to put a capo on the first fret, loosen the strings so they get flappy enough to lift the bridge off the posts slightly, then turn the set screws with the hand.
I use method 2. If I use a screwdriver to turn the set screws, even when the strings are loosened, this will cause screw slot damage. Little dents will happen. The exact same thing would occur if I used a flathead screwdriver to adjust the stop piece height on a Gibson Les Paul like this one. Sure, those big slotted stop piece screws make it appear you can just loosen the strings, stick a screwdriver in there and make your adjustments. I would never do that because again, that will cause screw slot damage. On a Les Paul, I would capo the first fret, loosen the strings enough to where I could slide the stop piece out, turn those screws by hand to make my height adjustment, then put the stop piece back and tune up the guitar again.
In either situation whether it's a bridge or stop piece height adjustment being made using method 2, strings have to be real loose to make said adjustment.
This is where a mistake can be made almost too easily. And I made the mistake I'm about to describe.
After I loosened the strings on my guitar, I somehow managed to bend the 6 string (as in the low-E string) in such a way where it developed a small permanent bend in it - but - it was so small and so slight that I didn't even see it.
I finish my bridge adjustment, tighten the strings to pitch, then come to discover I have some nasty fret buzz at the 11th fret on the 6 string and only the 6 string.
At this point I freak out. My first thought is that I've not even had this guitar 6 months and it's already developed a high fret.
I grab a metal dowel from my fret leveling kit because I needed to confirm how bad the high fret was...
...only to find I didn't have a high fret at all. The frets are all leveled just as they should be. There was nothing to fix.
At this point I'm confused. There's definitely a buzzing problem on the 6 string at the 11th fret.
I take a closer look at the 6 string, which I had loosened to check for a high fret, and then I saw it. A small "hump" in the string, exactly at the point where the fret buzz was happening.
The next day, I bought a new pack of guitar strings, put them on and the fret buzz was 100% gone.
Was it my fault?
I'm saying it is even though it might not have been.
I think what happened is that I somehow put a bend in the 6 string when I loosened it along with the others to made a bridge height adjustment.
However, it is also true that guitar strings that sit on a store shelf a long time can exhibit the same problem. Even though the strings are obviously coiled when in the pack, if coiled too long they may get malformed and never uncoil correctly.
It is possible I may have just had a bad pack of strings where the 6 string had a malformation in it. As noted above, I didn't even see that little "hump" in the 6 until I looked very close at it.
Fortunately, I had the proper metal dowel to confirm my frets were fine and that it was the low-E string that was the problem. That leveling kit saved my butt yet again. I didn't have to level any frets, but having those dowels confirmed 100% that the fret wasn't the problem.
This short video below shows how to use the dowels from the kit to check for high frets. I can assure you that the kit is worth buying just for the dowel set alone. The video says the kit has 2 dowels but it actually comes with 3. The third dowel is extra short for specifically checking close-spaced frets above the 12th on electric guitar necks.