During early stages of the game's development, Axel Stone and Adam Hunter had different names. Axel's name being Hawk and Adam's name being Wolf.
Contributed by Shiny Blazeikin
In a two player game, at the final boss, Spoiler:if one player answers yes and the other answers no when Mr. X says "Well, you found your way here. Would you consider becoming my right-hand man?", the two players will fight against each other. This is the only way to access the "bad" ending, in which the player takes over the syndicate.
Contributed by KidDivinegon