Sarah sees an older lady who is wearing glasses and has her hair styled in a bun. Sarah guesses that the lady is her school's librarian because she fits Sarah's librarian stereotype, even though there are way more teachers than librarians at her school. What has Sarah fallen prey to?

Association Fallacy

Explanation:

Sarah has "fallen prey" of an association fallacy. A very common error in our society. The association fallacy is a logical one, and it means, in simple words, that someone assumes that one person belongs to a certain group by simply looking at the other person's appearance.

For example: Besides the very good example given in the question there are thousands or million of other situations in which we can see this happens: people often categorize other's by their looks, hairstyles, piercings, skin color, clothes, accent, etc.

Sarah, i would think, was prey of the most common falacies of all: the association falacy.