It suffices to show two different parse trees for string cacac. Here they are. 

               S                       S
              /|\                     /|\
             / | \                   / | \
            S  a  S                 S  a  S
            |    /|\               /|\    |
            c   / | \             / | \   c
               S  a  S           S  a  S
               |     |           |     |
               c     c           c     c

Notice that the sequence of leaves, from left to right, is exactly the same (cacac) in each.