Transcription errorsTransposition errorsTranscription and transposition errorsRandom errors- Transcription, transposition and random errors
en.wikipedia.org/wiki/Check_digit#Design
The IBAN check digits aren't there to hinder "modifications". They are a means to detect errors in typing or transfer. For "random" errors, an average of one out of 97 will go undetected.
stackoverflow.com/questions/43260557/iban-checkdigit-effectiveness#comment73591887_43260557