"In Shakespeare's Romeo and Juliet, Juliet's mother rejected her, because of the power of Lord Capulet, and Juliet rejects the Nurse in the same scene, calling her a 'wicked fiend.'" This statement:

has a subject and predicate that don't agree in number.

includes a comma splice.

includes a pronoun with no appropriate antecedent.

*** includes a shift in person.

includes a shift in tense.

There's no shift in person -- all nouns and pronouns are 3rd person singular.

All subjects agree with their verbs.

There is no comma splice.

There's only one left ... do you see the shift in tense?

Oooooh, I see it now! "Rejected" and "rejects." Thank you! :)

This statement includes a shift in person. In the beginning, it describes Juliet's mother rejecting her, which is in the third person. Then, it shifts to Juliet rejecting the Nurse, which is in the second person. This inconsistency in person is what makes this statement contain a shift in person. To correct this, one could ensure consistency in person throughout the statement.