“Unbelievable” is hardly criteria for failure. In fact, it’s entirely immaterial, as long as the writer observes the only rule that matters about making sense: never pull the reader out of the vicarious experience.
Internal logic and consistency is important in helping readers stay in the vicarious experience.
I lean strongly toward the belief that readers want to believe, or at least suspend disbelief, and most will gloss over even glaring issues. I remember Michael Crichton’s translating earbuds in Timeline and after a moment of “Really?” I moved on and enjoyed the book immensely. (The movie, not so much.)