This bit of Arcana is well-known by older players, but it's such a goody that it deserves mention here.
The first version of Sage Owl (from Weatherlight) was illustrated by Mark Poole. Mark had illustrated another, more famous, blue card and decided to pay it respects in Sage Owl. The card? The original version of Counterspell. The painting in the background of Sage Owl is in fact of the same guy that is having his spell countered on Counterspell.