When dealing with agreement, teachers of Spanish as a foreign language and students usually focus on gender, considered the most difficult type of agreement to learn, probably because from the beginning, it requires much effort for the learner to know which is the inherent gender value of every noun than to choose the right number value depending on the context (although there are some morphological hints, gender is arbitrary and must be memorized).
second, agreement errors in texts can be identified and corrected straightforwardly by a native speaker, unlike other type of errors like article and preposition usage, for example, where annotator agreement may be problematic.