Gamey tactics

Gamey tactics are shorthand for "game the system", or put in another way, "cheat the system" by means of design flaws or poor AI. Gamey tactics almost always lead to ahistorical and extremely unrealistic results. Classic gamey tactics in HoI2 include:

Withdrawal & Flank: German withdrawal of troops from the French border, causing the French AI to take German territory, and then outflanking the bulk of the French army by capturing an abandoned maginot line.
Spamming infantry or IC: Spamming IC for years on end is extremely unrealistic, while creating a 100% infantry only army is highly imbalanced (though of course extremely effective in game).
Puppets: Large puppets, such as a Japanese puppet of Nationalist China, are highly unrealistic due to their huge amount of power.
The "Soldier of Fortune" approach: You can play the game as one country, then save the game, and return to the game as another country. You can pretty much always be where the action is.