Too many ships of one type were found on the board. This rule is encountered as the last contradictory step of an indirect logical sequence.
Example
The above board is the last step of an indirect logical sequence proving that (H,10) must be a ship segment. Assuming it were water would lead to the above board. Since there cannot be more than three destroyers (and there are four destroyers above!), this contradicts the possibility of (H,10) being water.