The music that plays in the opening is different in the American Release than in the Japanese release.
The boss of the third Mr. X stage, Metonger Z is a parody of the Super Robot created by Go Nagai, Mazinger Z.
Two of the Robot Masters, Wind Man and Knight Man, were actually designed by fans and included in the game as the result of a Nintendo Power competition.
After defeating Mr. X, there is a typo in his speech. He will say that he "Faild" instead of "Failed." This was later fixed in the Anniversary Collection.