It’s especially common to find errors of parallelism in sentences that list actions or items. If you list two gerunds, don’t switch and list a noun; if you list two nouns, don’t switch and list a phrase; and so on.

Ugly: Porter never liked drinking wine, eating cheese, or cocktail parties.

Awkward: The car is sleek, fast, and exhilarates whoever drives it.

If you’re writing a list in which the first item is preceded by a preposition or an article, be consistent with the rest of the list. Either leave out the following prepositions/articles (it’s allowed) or include the prepositions/articles before every item.

Watch out for errors of parallelism when you use either . . . or, neither . . . nor, both . . . and, not only . . . but also. The words on either side of those equations must use parallel structure.