You're right that "dumb" artillery shells should indeed be very predictable.
OTOH a missile or drone is more complex, something along the lines of a small automated aircraft. Hundreds if not thousands of moving parts, in this case possibly built and certainly maintained by Russia's finest.. Every now and then one of them will go wrong.....and it has to be said the same applies for most guided weapons used by most of the worlds armed forces...
So yes, a bollock having been accidently dropped is a real possibility.