Undesigned coincidences: subtle agreements between independent gospel accounts that are too incidental to be deliberate. Example: John says Jesus asked Philip where to buy bread (John 6); only Luke says the feeding was near Bethsaida; only John says Philip and Andrew were from Bethsaida. The connection (Jesus asked the local guys) only appears when you combine the accounts — no single author engineered it.

William Paley and John James Blunt (both Cambridge) developed this argument. Mark says crowds were busy "coming and going"; John says it was Passover (the busiest travel time). Neither explains the other, but combined they make perfect sense. Philip asked about bread because he's local to Bethsaida — but you need both John and Luke to see this. Too subtle for fabrication; best explained by multiple truthful witnesses.