It's very unusual to change a letter in a word or add one and not change the word's  meaning. But the clues below suggest two words which seem to do this. 

1. Can you think of two words which differ by only one letter and both mean to shake?

2. Can you think of two words, the second adding a single letter at the end, and both mean a side of something?

Answers:

 

1. QUAVER - QUIVER

2. FACE - FACET

 

Credit:
Joseph Appel