A spokeswoman for the building society confirmed that a problem in the credit card authorisation software, implemented in a routine security upgrade last August meant records of the transactions involved did not go through to payment network Link, so customers' accounts were not debited.
"It was a problem with the software itself, not the way it was put in," she said. "No heads have rolled - everyone is still employed."
Despite the fact it took five days to fix the problem, the spokeswoman said, "The problem was fixed pronto." It just "took a while" to discover the problem, she added.
She declined to comment on whether the building society would be suing the supplier but confirmed that it was still using Alaric software.
Alaric's managing director Mike Alford refused to comment on the nature of the firm's discussions with Coventry but questioned the building society's version of events.
"I do not agree with what they are saying. The software Coventry got from us has gone live in other building societies without any problems," he said.
IT barrister Stephen Mason said, "I would not be surprised if the two companies are locked in a battle as to who should pay compensation.
"It depends on the nature of the contract between Coventry and Alaric as to who is liable for what, but with Coventry being quite a big building society I would imagine they would have looked at these matters quite closely."