[MarkusQ]

> parallel construction errors, like "This product is fast, lightweight, and won't break the bank!"

I'm failing to see the error. That seems like perfectly sound, vernacular English.

[sowbug]

The first two of the three are adjectives, each connected to the subject by the one "is," and the third is a verb phrase not using the "is." Ideally they'd be all adjectives using the "is," or all phrases supplying their own verbs.

Not the worst error in the world, but it stands out in LLM text that is otherwise remarkably nit-free.

[MarkusQ]

But is it even an error? You are parsing it as a single list, but it could just as well be parsed as "subj ((is {a,b}) and vp-predicate)".

I guess you could argue that the first list needs an "and"? That's fair I suppose.