US Open champion Rory McIlroy ended his opening round at the Irish Open with a double bogey and then became embroiled in an argument on Twitter.
McIlroy was four under after 10 holes but bogeyed the 11th and then found the water at the 18th as he carded a one-under-par 70 at Killarney.
And he then sent an angry tweet to former European Tour player Jay Townsend, who had criticised his play.
Replying to the Radio 5 live pundit, McIlroy told Townsend to "shut up".
McIlroy quickly replied: "Shut up ... you're a commentator and a failed golfer, your opinion means nothing!"
And when Townsend - runner-up to Sam Torrance at the 1993 Heineken Open in Spain after the final round was cancelled for snow - responded that he stood by his comments, McIlroy added: "Well I stand by my caddie."
The youngest winner of the US Open since 1933, McIlroy has had JP Fitzgerald on his bag since the middle of his first full season as a professional in 2008.
And in a later interview he stood by his forthright response, saying: "He's been having a go at JP every now and again since then and this was the first time I've responded - it was the straw that broke the camel's back.
"Now I've blocked him on Twitter so I won't be reading anything more."
Jeev Milkha Singh leads the field on eight under, with Christian Nilsson and Marcel Siem tied for second on on five under.
Newly crowned Open champion Darren Clarke finished two under after a 69 but Graeme McDowell carded a one-over-par round.
McIlroy began his day brightly with birdies at the fourth, sixth, eighth and 10th holes before falling down the leaderboard.
"I put myself in a good position but my second nine was pretty disappointing," said McIlroy, who put his tee shot at the last into a bunker then found water with his second as he went for the green.
"I played some scrappy golf on the way in and a six at the last is hard to take, but I'm still not too far away.
"The course is in great condition and has been set up really nicely. The greens are a little slow but at least you can be aggressive with your putts."
Colm Moriarty, Ross Fisher, Richard Green. Niklas Lemke, Steve Webster, Lloyd Saltman, Bernd Wiesberger and Soren Hansen are all in the clubhouse on four under.
Fisher produced steady golf to end with a bogey-free round while surprise package Moriarty posted six birdies and two bogeys.
Clarke recorded two birdies and a bogey in his opening nine holes but went back to level par with a bogey five at the 13th.
Birdies at the 16th and 17th holes saw him move to two under.
McDowell carded two birdies and three bogeys as he struggled to an unconvincing 72.