These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Good Bad Bugs: The AI does not understand the concept of attrition. It's entirely possible to lure a large army into your territory, Scorch the Earth in that province with a fast / sacrificial unit, and let that (or the winter) whittle the enemy army down to almost nothing.
The flipside is that naval attrition has to be disabled for the AI, leading to things like Ottoman territories in the Baltic, or Castille owning Finland.
Naval attrition in European waters is nigh-nonexistent anyway.
If the player starts the game anytime later then 1399, the game would remember all changes that happened since that year and mention them in the country's history log. Start the game as the USA anytime after 1797 and read how George Washington's period was only remembered for his drunkenness!
Memetic Mutation: On the forums, someone started a thread titled "Can Poland into space?", itself derived from Polandball. This has become a minor meme, with the Strange Screenshots thread containing many instances of nations claimed to "into space" - apparently it's the new word for "blobbing". The backlash is summed up by this image◊.