An Easter egg is a special hidden feature/message that a creator of a game implements.
There are very few hidden Easter eggs in the game.
List of Easter Eggs
All your money are belong to me
When robbing eight/twelve the response to rob the cashier is "All your money are belong to me". This is a reference to a popular "engrish" mistranslation in Zero Wing.
Removed Easter Eggs
These Easter eggs were once in the game, but can no longer be found.
Floating HouseIt is known that this floating house was actually used for a screenshot for the official game page, but the creator, Firebrand1, left it there as an Easter egg, or at least we think. It can be found near the edge of the map. Many reported that the floating house displayed different home blueprints at different times, however the house always had default painting.
The house glitch before & after: