If the bank does not accept a transfer due to an error it gives a error message which is silently ignored. Gnucashs XML file as well as the account view look like an successful transfer, and the user will get aware of the failure not before the bank statement shows that nothing was transfered, unless he carefully checks the log in the "Onlinebanking connection window".
The only difference between an successful and a failed transfer are two lines

successful:
HBCI: 0010 - Nachricht entgegengenommen. (M)  (eng: message accepted)
HBCI: 0010 - Auftrag entgegengenommen. (S)    (eng: order accepted)

failed:
HBCI: 9050 - Die Nachricht enthält Fehler. (M)
HBCI: 9210 - Der von Ihnen erstellte Auftrag ist für diese Kontoart nicht zulässig. (S)

(in english something like: 
HBCI: 9050 - The message contains errors
HBCI: 9210 - the order you submitted is not allowed for this type of account)

Another example for the second line:
HBCI: 9210 - Die Empfänger IBAN ist nicht korrekt. (S) 
(eng: recipients IBAN is incorrect)