Errors in sentences with interpolated coordination, in which a phrase providing additional information is inserted but punctuation and/or words that provide complementary structure are omitted or misplaced, are frequently made but easily avoided, as explained in the discussion and demonstrated in the revision following each example below.
1. It is widely regarded as one of, if not the, greatest films ever made.
The flaw in this sentence is simple to detect—simply omit the parenthetical phrase: “It is widely regarded as one of greatest films ever made.” If the base sentence is syntactically flawed, then the same sentence, with interpolated wording, is also incorrectly constructed.
To repair the damage, word the base sentence so that it stands on its own (“greatest films” must be preceded by the article the within the base sentence), and revise the parenthetical phrase so that it complements the corresponding phrase in the base sentence (“greatest films” cannot serve both the base sentence and the parenthetical phrase): “It is widely regarded as one of the greatest films, if not the greatest film, ever made.” An alternative revision that interpolates the parenthetical phrase early but is not as elegant is “It is widely regarded as, if not the greatest film, one of the greatest ever made.”
2. Mobile apps perform the same or better than they did a year ago.
Here, the phrase “or better than” is not technically a parenthetical phrase because it is not punctuated (though some writers would do so), but it serves the same function—it interpolates additional wording into the base sentence, in this case “Mobile apps perform the same they did a year ago.”
But notice the flaw here: The comparative phrase “the same as” is missing a word, so revise as shown here: “Mobile apps perform the same as or better than they did a year ago.” Without this insertion, the erroneous implication is that than serves as a conjunction for both same and better when, according to grammatical rules, it supports only the latter word.
3. Membership or inviting support for the organization is a criminal offense that carries a sentence of up to ten years.
A missing word is the problem here, too, but this time it is an absent preposition—membership requires its own preposition so that, similar to the problem in the previous example, it is not misunderstood to share for with “inviting support”: “Membership in or inviting support for the organization is a criminal offense that carries a sentence of up to ten years.” (Again, the interpolation “or inviting support for” could be treated as a parenthesis with bracketing punctuation, but doing so is unnecessary.)