Celtic famously lodged a bid for Lee Tomlin during a Peterborough match.
Celtic - under manager Neil Lennon at the time - had been linked with Posh midfielder Lee Tomlin and made a bid for him during a Peterborough match in a move which was condemned by United's people at the time.
MacAnthony took to Twitter to air his frustrations, writing: "I used to be a Celtic fan. Not after today! What a day" and Peterborough's then-Director of Football Barry Fry also slammed the brazen move by Celtic (The Mail).
It is three years on from that saga and the Irish owner admits the incident still disappoints him, though reveals that he has remained a Celtic fan despite it all.
"I’ll never forget Celtic trying to sign Lee Tomlin from us a few years ago. They put a bid in while we were actually playing a game," he told The Scottish Sun.
“Celtic are a club close to my heart, but I was disappointed with them that day. Not just their timing, but also their offer.”
Tomlin reportedly wanted to join Celtic at the time, but it has worked out well for the 27-year-old.
He left Posh for Middlesbrough - then in the Championship - the following summer and secured a move to Premier League new boys Bournemouth one year later.
Tomlin, who spent the second half of last season on loan at Bristol City, re-joined the Robins on a permanent basis in July.
Barry Fry (L) - Peterborough United Director of Football with Chairman Darragh MacAnthony