I don't know exactly whats the case with OP here but lets say you got a number of beneficiaries attached to your account , I use Internet banking and I just have to select the beneficiary from the list to complete the transaction once I mentioned the amount and sometimes a beneficiary with whom you just had few transaction get selected , just normal human error.

 You need to add account number and account holder's name for the first time when you add a beneficiary to the account after that just hit the radio button and thats it, at least this is how it works in the internet banking software of bank I use.