What kind of errors are undetectable by the checksum?

Answer

At least three types of error cannot be detected by the current checksum calculation.

First, if two data items are swapped during transmission, the sum and the

checksum values will not change. Second, if the value of one data item is increased

(intentionally or maliciously) and the value of another one is decreased (intentionally

or maliciously) the same amount, the sum and the checksum cannot detect

these changes. Third, if one or more data items is changed in such a way that the

change is a multiple of 216 − 1, the sum or the checksum cannot detect the changes.