A top Post Office lawyer has said he and others "missed" a key finding in 2014 that it was possible to remotely access Horizon software.
Rodric Williams told the Post Office inquiry that repeated assurances this was not possible had been inaccurate.
This included assurances to independent investigators and the BBC.
A key assertion in Horizon scandal court cases over many years was that remote access to the software was not possible.
The Post Office asserted in hundreds of wrongful prosecutions that sub-postmasters must have been to blame for discrepancies in accounts.
And in certain key cases, such as the landmark Bates vs Post Office, the organisation insisted that the software could not be accessed remotely.
However, a report by Deloitte, which had been coordinated by Mr Williams, stated in 2014 that in fact it could be.
When asked by inquiry counsel Jason Beer how Mr Williams had missed this, despite it being a key concern, he said: "I genuinely don't recall what was happening at the time."
In trying to reconstruct his thought processes, Mr Williams said "it's hard to explain" why he missed it.
"It was a theoretical possibility, that had controls around it, to mitigate the risk, which was small, and was being investigated further, and so, I think off the back of that, it clearly went from my mind, because I don't recall it popping up when we were being asked later [about remote access]."
Separately, Mr Beer asked Mr Williams about alleged instructions to shred documents related to the scandal in 2013 by the then head of Post Office investigations, John Scott.
This was allegedly to prevent disclosure of the documents in any court proceedings.
Speaking to the inquiry in 2023 about the allegation, Mr Scott said: "I have no recall from that period, from 10 years ago, but that is not me. That's just not me, what I would say or ask."
On Friday Mr Williams said the allegation was "serious" but that he didn't think any documents had been shredded - but he didn't know whether that was correct.
Mr Beer asked him if there was ever any investigation by the Post Office into the shredding instruction.
Mr Williams responded: "I do not know."
Mr Beer asked whether the Post Office had considered reporting the matter to the police.
Mr Williams responded: "I don't believe so, no."
When the BBC asked the Post Office whether it had investigated this, it said that was "a matter for the inquiry".
"Our first priority is always to assist the inquiry in its role to establish the truth," a Post Office spokesperson said.
"It's for the inquiry to reach its own independent conclusions after consideration of all the evidence on the issues it is examining."