Few drops of HCl is added to acetic buffer. The pH is maintained constant by

${\text{CH}}_{\text{3}}{\text{COO}}^{-}+{\text{H}}^{+}\to {\text{CH}}_{\text{3}}\text{COOH}$

Then the ${\text{H}}^{+}$ions added to the buffer are removed as un dissociated ${\text{CH}}_3\text{COOH}$. Hence the pH of the solution does not change.