An error?

There are two types of grammar: Descriptive, which describes what is customary, and prescriptive grammar, which prescribes what should be. A tension between the two systems is inevitable — and healthy; it keeps us thinking about what we’re saying and writing.
Allowing mob rule at the expense of some governing of composition is madness, but a diction dictatorship is dangerous, too. As with any prescription, an overdose is contraindicated. Here are some hard pills to swallow for language mavens who require a strict adherence to rigid syntactical patterns at the expense of, well, language: (from Daily Writing Tips)

What doesn't constitute an error in language use? Click the above hyperlinked text to find out.