When I was 16 years old, a teacher pointed out that my placement of the word "only" in a sentence changed the meaning from what I had intended. Ever since that episode, I have paid more attention to the placement of the word.
To see the importance of the placement of "only", consider this graphic that I spotted on Facebook [via JS]. Try putting "only" in each possible location and then consider how the meaning changes as you move the word from place to place.