I thought this was pretty interesting. I wonder if there is a hidden meaning behind this, rather than just a goof.
Snippet from article:
Art restorers have discovered the figure of a devil hidden in the clouds of one of the most famous frescos by Giotto in the Basilica of St Francis in Assisi, church officials said on Saturday.
Complete Article on YAHOO: Devil found in detail of Giotto fresco in Italy’s Assisi