Nerf instability is the state where unbalanced RPG designs devolve to:
essentially they make all skill investment risky as any patch can nerf or rework these skills. Especially common when game company monitors the use of various character builds and tries to achieve perfect balance between classes /PvE/PvP(at expense of skill stability) with constant adjustment and rebalancing:this maintains the nerf instability.

Similar but wider concept is beta instability: wider changes from major patches to entire classes and game mechanics. often seen in beta tests.