The Welsh Government says it has acted to restrict professional lobbyists' access to ministers after details of a meeting emerged.
Economy minister Ken Skates met a broadband company in the assembly last year.
Lobbyist Daran Hill was also there, despite assurances that ministers do not have formal meetings with commercial lobbyists.
A government spokesperson said a "misunderstanding" was to blame.
She said that the government had "changed our processes" to stop it happening again.
Former First Minister Carwyn Jones said in 2016 that ministers do not have formal meetings with lobbyists.
The meeting with CityFibre, a London-based company that builds fibre optic networks, was arranged by Labour backbencher Jack Sargeant.
Answering a freedom of information request, the government said someone from Mr Hill's company Positif Politics - now known to be him - "had not been invited to the meeting, but arrived with other scheduled attendees".
'Not in line with procedures'
Unlike Scotland and Westminster, lobbyists in Wales do not have to sign an official register.
Last year, the assembly's standards committee said instead of introducing a register, a group of AMs should voluntarily reveal their meetings with lobbyists and interest groups as part of a pilot scheme.
A government spokesperson said: "In June the then economy secretary met an assembly member and an external company to discuss a broadband issue.
"It became apparent that one of the party had brought a guest with them who had not been on the attendee list.
"We recognise that this person's attendance at the meeting was not in line with usual Welsh Government procedures and have since changed our processes to prevent such a misunderstanding from happening again."
Mr Hill said: "The rule which I appear to have transgressed has never been properly laid out or communicated externally.
"The former first minister made this policy without any attempt to define what a lobbyist is or what a meeting is.
"I regret putting Ken Skates in an awkward position, he had no idea I would be at the meeting.
"If I had known what the opaque rules actually were, I would not have gone."
Jack Sargeant said: "This issue has since been dealt with."
CityFibre has been asked to comment.
What is a lobbyist?
By Daniel Davies, BBC Wales political correspondent
Wherever there are governments, you'll find people trying to influence government policy.
That's what a lobbyist does - and they can represent all sorts of organisations, charities or businesses.
On this occasion, a "misunderstanding" has led to the Welsh Government tightening up its procedures to keep lobbyists at arm's length.
But Daran Hill says those procedures - which stem from an assurance by Carwyn Jones in 2016 that "ministers do not have formal meetings with lobbyists" - should have been clearer in the first place.