My question arises from this one, where OP asks whether he should use the singular or plural verb form after "the title, as well as the tone,"
As luck would have it, when I searched Google Books for exactly that text, I found...
The title as well as the tone of this volume errs, perhaps, in being unduly modest.
...and...
The title, as well as the tone, of the novel derive from Lady Molly Jeavon's household
At the risk of having this question closed as just peeving, I feel that the first one should have been plural "err", because without the commas it seems to me both "the title" and "the tone" have more or less equal status as "joint" subjects of the verb, making it plural.
On the other hand, I feel the commas in the second example effectively demote "the tone" to "parenthetical" status (the clause is almost incidental). Since this encourages us think of "the title" as the primary subject, it should have been singular "derives".
I'm no great fan of "grammatical rules", but it seems weird to me that the only two relevant examples for this specific wording should both (in opposite ways!) contradict my inclinations.
What's going on?