Buffer Capacity
No buffer has an unlimited capacity. ie; buffers can only absorb so much abuse before they are destroyed. The capacity of a buffer is the amount of acid or base it can handle before the pH of the solution changes drastically.
 
If you add enough strong acid to neutralize all of the buffers basic component, then additional strong acid will make the pH drop rapidly. The same applies for a strong base and the buffers acidic component. The buffer's pH is a function of it's pKa and the ratio of concentrations of anion and acid, but the buffer's capacity depends upon actual concentrations.