What errors does casting out nines fail to catch?

Any error that's a multiple of 9 — including every digit transposition — slips through, because rearranging digits leaves the digit sum unchanged.

• 1234 and 1324 both have digit sum 10 → digital root 1. Casting out nines can't tell them apart, so it misses that swap.
• An error of exactly ±9, ±18, ±27… also passes.

So a matching nines-check is necessary but not sufficient — it catches many slips (roughly 8 in 9 random errors) but is blind to transpositions, one of the most common human mistakes.