I always figured that animators embedded jokes or images into cartoons that would slip by viewers. I mean, they draw cartoons; they've got senses of humor.
This site meticulously lists hidden gags in the Looney Tunes and Merrie Melodies cartoons. Granted, they're pretty innocuous. Many of the catches are names of animators or producers inserted into business signs or book titles. The site explains the relevance of the gags you'd miss if you weren't obsessive about finding them, like the site administrator obviously is.
Some gags are more clever. Of course, I'm partial to this one:
The squirrel is inspecting acorns and stamping them, but he happens to be stamping the hebrew word "kosher" onto them.
BTW, I found this link through the site Drawn! which is a weblog for artists, illustrators and cartoonists. It usually has some neat stuff on it.