The Tomato Incident is an event that took place in the Hey Arnold! universe.
It was a small battle between the neighbours of 44-49th street and the British. The event was only mentioned once in Hey Arnold!: The Movie.
It seems that after the Pig War, the Americans still had to pay taxes to the British on a lot of stuff. One day the British raised the tomato tax to a nickel (from 2 cents to 8 cents) which caused them to be at war with the neighbourhood. While the British used guns as weapons, the neighbourhood used British tomatoes as their weapons.