Back when Skyrim released people realized there were certain bug jars in the game with strange inscriptions on the underside of their lids. People have been stumped for years as to what they meant, and there are multiple theories to what it was about.
Here's a summary about it and prevailing theories: http://elderscrolls.wikia.com/wiki/Forum:Skyrim:Bug_In_A_Jar_Conspiracy
Some think they're clues to in-game actions you have to perform to unlock something. Some think they are a hint about the next game. (The games Oblivion, Morrowind, and Redguard had hints about future Elder Scrolls games). Others think they're just an easter egg left by the game devs. What do you think?