What the check digit catches
The ISO 6346 mod-11 check digit detects every single-character error — one wrong letter or digit anywhere in the first ten characters — and most transpositions, where two adjacent characters are swapped. That covers the large majority of real keying and transcription mistakes.
The blind spot
The check is blind to a substitution whose two characters have values that differ by a multiple of 11, because both contribute the same amount modulo 11. The classic pair is the digit 1 (value 1) and the letter B (value 12): swapping one for the other leaves the check digit unchanged. A transposition inside the serial can also survive the check. When the first ten characters are wrong but still produce the printed check digit, only comparison against another source — a booking, a load list, the physical box — will reveal it.
Why O and 0 are never swapped for you
The most common real-world errors are the look-alikes: the letter O read as the digit 0, the letter I as the digit 1, or the letter B as the digit 8. Checkdigit never substitutes one for the other on its own. If a serial contains a letter where a digit belongs, it says so and keeps your original entry, rather than quietly "fixing" it into a different container number.
How Checkdigit reports a failure
When a number fails, the calculator shows the expected check digit and the number it would form if the first ten characters are correct — but it never assumes which end is wrong. Verify the whole number against your source, because the error may be in the body rather than the check digit.