Consider the sentence “The site’s editor Mary Smith wrote an opinion on the topic.” The lack of supporting punctuation in the identification of the writer of the opinion is an error.
This mistake is common when appositives are involved. An appositive is a word or phrase that is equivalent in meaning to another; in this case, “the site’s editor” and “Mary Smith” are different ways of referring to the same person, so they are appositives. (Similarly, in “The car, a brand-new Tesla, attracted much attention,” “the car” and “a brand-new Tesla” are appositives.)
“The site’s editor, Mary Smith, wrote an opinion on the topic” and “Mary Smith, the site’s editor, wrote an opinion on the topic” both present additional information that is not essential to the sentence (and therefore is set off parenthetically by a pair of commas). Each one also unequivocally identifies Mary Smith as the sole editor of the site. (However, whether that means she is the only person who edits content on the site or she holds the specific position of editor and supervises one or more assistant editors is not certain; it would be better, if the latter is true, for her to hold a more distinctive title, such as “editor in chief.”)
By contrast, the original wording is flawed, in that the appositives, or equivalent phrases, “the site’s editor” and “Mary Smith,” butt up against each other without intervening punctuation. (In the preceding sentence, appositives and “equivalent phrases” are themselves appositives.)
The similar-looking but distinct construction in “Site editor Mary Smith wrote an opinion on the topic,” however, is correct, in that “site editor” is a job description that is essential to understanding Smith’s role in writing the opinion, rather than a parenthetical explanation that can be omitted. This version, though, also makes it unclear whether Smith is the only editor; is she the editor, or an editor? To indicate the latter, “Mary Smith, one of the site’s editors, wrote an opinion on the topic” or “Mary Smith, a site editor, wrote an opinion on the topic” is better.
The original problem is related to that inherent in a sentence such as “John’s sister Jane is getting married.” The sentence may not accurately reflect how many sisters John has. As it is written, Jane is only one of two or more sisters; the lack of a comma between sister and Jane indicates that her name is essential information: The sister of John who is named Jane is getting married. “John’s sister, Jane, is getting married,” on the other hand, includes an optional parenthesis: John’s sister, whose name is Jane, is getting married.
Depending on context, appositives may or may not be set off from each other by commas or other punctuation.