Write down the mistake's mechanism
An error note is more useful when it explains how the mistake happened. Writing wrong answer preserves little guidance. Writing that you applied the plural rule to the wrong noun identifies a specific point to check next time, turning an incorrect result into a reusable correction.
Go deeper
Preserve the correction, not just the failure
After getting an answer wrong, it is tempting to copy the correct answer and move on. That may leave the original misunderstanding untouched.
An error note identifies the mechanism of the mistake. Include the incorrect move, the reason it failed and the condition that calls for the correct move.
For a sentence-editing task, the problem might be choosing a verb to match the nearest noun instead of the sentence's subject. That explanation provides something specific to inspect in future sentences.
The note need not become a record of every slip. A typing accident may require little analysis. Recurring or revealing mistakes deserve more attention because they expose a rule, distinction or step that has not yet become dependable.