Knowledge as Justified True Belief: Repaired

To fix the [justified true belief] definition of, simply alter [justified] to [stable]. Lack of justification is one source of instability but hardly the only one. 

A guess is unstable because you're apt to second-guess yourself, even if the guess is accurate the first time.

A Gettier case is unstable because even though one of your beliefs is accurate, 99% of apparently identical beliefs are inaccurate and if you interact with them you're also apt to change your true belief. (Camouflage is possible but that doesn't mean knowledge is impossible.)

A false belief is unstable, no matter how well justified, because you're apt to learn better.