There are times while writing where we have the tendency not to stay in a present tense. This is a common error for many writers including myself and that is whyWilliam Strunk and E.B.

Example: 1A
Chance prevents Friar John from delivering Friar Lawrence’s letter to Romeo. Meanwhile, owing to her fathers arbitrary change of the day set for he wedding, Juliet has been compelled to drink the potion on Tuesday night, with the result that Balthasar informs Romeo of her supposed death before Friar Lawrence learns of the non delivery of the letter. (Strunk and White 31)

Note: Using the Romeo and Juliet scenario a reader can be confused on what events had or had not occurred. If the writer is changing their tense throughout their work, it leads to uncertainties for the reader to rely on an author being a reliable source.