There are really 2 parts to this slip up.
In the scene where Cameron has just gotten off the phone to Mr Rooney, Ferris kicks him on the backside.
1) They start an argument, where ferris starts off by saying "Wheres your brain?" This continues for a short time, and then Cameron says "I asked you first", when it was clear that Ferris had started it
2) In the same scene, just a short while later.. ferris says "Now i didnt hit you, I lightly slapped you" and cameron says "you hit me."
It was quite clear to me that ferris had kicked him, rather than hit him.