The Devil is in the Details, In This Case, Literally

The picture below is 1 of the frescos painted by Giotto di Bondone in the Basilica of St Francis in Assisi. This fresco is  fresco number 20 in the cycle of the scenes in the life & death of St Francis. It was painted by Giotto in the 13th century. Recently, Italian art historian Chiara Frugone made an interesting discovery, a devil hidden in 1 of the clouds. According to the report it is difficult to see from the floor but is clearly visible in close-up photographs.

If you cannot see it in the above photo then here is a close-up of the section with the devil:

It is a profile of a figure with a hooked nose, a sly smile, & dark horns. According to Sergio Fusetti, the chief restorer of the basilica, Giotto probably never wanted the image of the devil to be a main part of the fresco. He may have painted it in among the clouds "to have a bit of fun."

Fusetti added that Giotto may have painted it to spite someone he knew by portraying him as a devil in the painting.  

There is some controversy as to whether or not Giotto actually painted the frescos. That aside, whoever did clearly intended to have a bit of fun at a friend's expense.

Source: Devil found in detail of Giotto fresco in Italy's Assisi