Unfortunately you can never rule out simple human error, where the vicar/clerk forgot to make the entry in the register.
There's a known example of that in my local churchyard, where there's a grave and headstone, and people who remember the funeral taking place, but the burial isn't in the register. This came to light when someone was recording the MIs and making a plan of the graves, but there could easily be others in earlier unmarked graves that went unrecorded.