The best answer to this is to sell yourself to the interviewer. Make it look like your going through a list of times this has happened in your head, and then make sure whatever one you have is pretty decent. Mine was " Well as a grocery stocker for !@@#@$@, it is required for us to do a set, scheduled task load every day, however, i had one case where i had finished all my scheduled work for that day, and decided that i would help the closing manager with conditioning and cleaning the store. When i finished, the closing manager said that we needed to get started with conditioning and i just looked and her and said 'Done.' In doing so, not only did i win her praise, but it felt good knowing i made her work load a little easier." Simple as that ;D