A Bar at the Folies Bergere, 1881-82 by Edouard Manet, Courtauld Institute, London.
Manet's last major painting. At first glance you see the bored looking barmaid staring off into the distance. Upon closer inspection we can see all the activity of the bar going on in the mirror. Including a patron whose presence is not doing anything to improve her mood - perhaps he caused the mood? Those tangerines just jump out at you, don't they?