The fallacy of grammatical analogy, also known as the fallacy of false analogy, occurs when someone assumes that because two things are similar in one aspect, they must be similar in other aspects as well. This fallacy disregards relevant differences between the two things being compared.
Example:
Person A: "I heard that eating chocolate is good for you because it contains antioxidants. Therefore, eating ice cream must also be good for you because it contains milk."
Person B: "That's a fallacy of grammatical analogy. Just because chocolate and ice cream share the similarity of containing milk, it doesn't mean they have the same health benefits. Chocolate contains a higher concentration of antioxidants, which is the main reason it is considered beneficial."