Never seen this happen before, the string has become (semi)-permanently kinked. It still tunes up, but gradually drifts out, and the kink makes nasty fret buzzing. Figured I'd take a pic before I change the string. Little hard to see as the kink is directly over the fret.
(https://sphotos-b.xx.fbcdn.net/hphotos-ash3/622759_3642932758837_789305966_o.jpg?)
Damn you fret hard.
I've seen it happen before, but usually from some extreme accident putting pressure under the string (like it falling off a stand) or some mishap while changing strings. You've got to change it, once they're bent like that they're pretty well fucked.
The only way I could see that happening from fretting is if you were fretting behind and in front of the fret at the same time, with really high action on really big frets.
if i leave the strings on my crap-o-caster for a long time, it will eventually do that. we are talking months and months of daily playing, which never happens any more.