It only mentions a few bugs out of the hundreds that were discovered (a full list hasn't been published), but it appears that the bugs weren't the only issue:
> In 2015, for instance, the Post Office told the House of Commons inquiry: “There is no functionality in Horizon for either a branch, Post Office or Fujitsu to edit, manipulate or remove transaction data once it has been recorded in a branch’s accounts.” This was untrue, and the Post Office admitted as such four years later during a high court case.
> In fact, staff at Fujitsu, which made and operated the Horizon system, were capable of remotely accessing branch accounts, and had “unrestricted and unaudited” access to those systems, the inquiry heard.
It's truly a scandal that no-one in charge at the Post Office or Fujitsu has been prosecuted for providing false testimony that led to so much misery and death for the sub postmasters. The MET has finally started to investigate fraud and perjury charges which seems like they only bother investigating crimes that have been made into a popular four-part series.