Were all books of the New Testament written in perfectly correct Koine Greek?

Revelation is notorious for its grammatical errors; google Revelation and Solecism (fancy Greek for “bad grammar”) or Barbarism (fancy Greek for “L2 Greek”). You’ll see lots of attempts at explaining it, from the straightforward “he barely spoke Greek” to “he was cutting and pasting bits of the Septuagint without adjusting the grammar” to “there’s a deeper theological reason for it”.

Someone recently did a PhD on it, which seems to get a bit too theological for even theologians, as seen in this review of Morphological and Syntactical Irregularities in the Book of Revelation.