Question

The graduate students filled out the necessary applications, and submitted the required references.How is the comma used incorrectly in the sentence above?

Answer 1

A comma is not needed if it's only comparing 2 things and it has an "and" present. 

Answer 2

a comma is not required when a list or series includes only two things.
The two things here are predicates, as it is a compound sentence.
rather than using comma, it's better to just use "and"

Hope this helps