What is buffer capacity and how can a buffer "run out"?
If strong acid is added to a buffer system, conjugate base removes it.  Eventually, the concentration of conjugate base falls to such a low level that its effect on pH can be neglected.  At this point, the solution behaves more like a weak acid solution.  If even more acid is added, the system behaves like a strong acid system in terms of calculating pH.

If strong base is added to a buffer system, conjugate acid removes it.  Eventually the concentration of conjugate acid falls to such a low level that its effect on pH can be neglected.  At this point. the solution behaves more like a weak base solution.  If even more base is added, the system behaves like a strong base system in terms of calculating pH.

In the equation you wrote, $$\ce{A- + H2O &lt;=&gt; HA + OH-},$$ think about the effect of adding acid.  Acid reacts with the $\ce{OH-}$, producing water and removing $\ce{A-}$ from solution.  The $\ce{OH-}$ and $\ce{A-}$ cannot be removed from solution but their concentrations can be driven to negligible levels by the addition of sufficient quantities of acid.