Is there an error in this sentence: "Donald and Isaac had no idea how the movie would affect them; they usually aren't very fond of romantic comedies?"

The sentence "Donald and Isaac had no idea how the movie would affect them; they usually aren't very fond of romantic comedies" is grammatically correct and clear in its meaning, which justifies the conclusion that there is no error present.

The structure of the sentence appropriately uses a semicolon to connect two independent clauses that are thematically linked. The first part discusses the uncertainty that Donald and Isaac feel about the movie's impact on them, while the second part reveals their general dislike for romantic comedies, providing context for their apprehension.

Each component of the sentence functions correctly: "Donald and Isaac" is a proper noun referring to two individuals, "affect" is used correctly as a verb indicating influence, and "them; they" effectively separates two clauses that can stand alone. Therefore, the statement that there is no error is accurate.