World number one Andrew Selby suffered another Commonwealth Games nightmare as he was stunned by Motherwell 19-year-old Reece McFadden in a fiery flyweight first-round clash in Glasgow.
The double European champion from Barry was deducted two points in the last round for losing his gumshield as the irresistible McFadden did enough to seal by far the best win of his short career.
Selby, who also boasts two World Championship medals in his collection, was also shocked in Delhi four years ago when he dropped a verdict to Amir Khan's brother Haroon.
Selby was gracious in defeat, saying: "Everything went wrong in the third round. As soon as the bell went, it all went out of the window.
"I had my gumshield specially made and I've worn in at every tournament I've been in in the last four years and it's never come out.
"I'm in the best shape of my life and I don't know what went wrong. Maybe he just took me by surprise and he deserved the win."
Selby's defeat was another major blow for the Welsh boxing team following the decision by Games authorities to deny accreditation to his team-mate Fred Evans.
But it was a significant moment for McFadden, who rose to the occasion quite brilliantly to out-hustle Selby in the opening round and maintain a tough momentum throughout.
Selby won the second to claw the scores back level and the messy last was rendered irrelevant by the points deductions, which effectively sealed McFadden's shock win.
Selby showed his frustration in the dying seconds by launching himself at McFadden, who celebrated wildly at the final bell.
The Scot, who will next face England's Charlie Edwards, said: "I've fought some tough boys but he was the toughest I've fought in my life.
"The gold medal is mine. I've just proved it against the world number one and I'll beat Charlie Edwards next. Hopefully he'll keep his gumshield in."
There had also been success for the hosts in the opening session as 6ft 6in super-heavyweight Ross Henderson outpointed Parveen Kumar of India to set up a clash with Joe Joyce of England with a medal at stake.
Joyce, who beat Keddy Agnes of the Seychelles, will start as favourite for their bout on Tuesday having won both of their previous meetings, but Henderson believes the home crowd support will work in his favour.
Henderson said: "Joe and me have fought twice before and they were two close fights, but fighting in front of this crowd in Scotland is brilliant and I want to go on and get a gold medal.
"Four years ago I was just there for the experience and I was already kind of getting ready for Glasgow. But I have been getting better every year since then and today it was fantastic to be boxing in front of all these people."
Joyce produced a workmanlike performance to ease past Agnes and got a taste of the likely atmosphere on Tuesday when he was booed out of the ring by a small section in the sell-out SECC crowd.
"I don't let it get to me and I don't even hear it, to be honest," Joyce said. "Obviously Ross will have the crowd behind him and it will be quite interesting to walk out to that so I am looking forward to it either way."
Joyce has beaten Henderson in both of their two previous meetings, flooring him en route to a points win in the 2011 GB Championships and outpointing the Motherwell man in Finland the following year.
Joyce's England team-mate Scott Fitzgerald made an impressive start at welterweight, wobbling St Lucia's Ron Bastien in the opening five seconds before stopping his opponent early in the third and final round.
Northern Ireland's Olympic bronze medallist Michael Conlan overcame a scare as he opened the bantamweight competition with a unanimous points win over Mathew Martin of Nauru.
Conlan's face was left streaked in blood after the third-round head clash but the small zigzag cut is not expected to compromise his future in the competition.
Team-mate Ruairi Dalton also progressed in the flyweight division with a points win over Christopher Katanga of Zambia, but there was disappointment for Scotland's Lewis Benson, who dropped a split decision to Bowyn Morgan of New Zealand.
Ballymena welterweight Steven Donnelly continued his remarkable path to Commonwealth Games redemption with a dramatic first round knockout of Pakistan's Hasan Arif.
Four years ago Donnelly lost in the first round in Delhi then was sent home in disgrace after breaking a team curfew.
The decision sent Donnelly's life spiralling out of control and his career appeared over as he gave up the sport for two years.
Donnelly said: "Delhi was a disaster and for a few years after that I never thought I would box again.
"I was in low, low places but I apologised to everyone and it took me months to get back into shape.
"I use what happened in Delhi as my motivation. I have got myself together and trained particularly hard and I am back here looking for a much better result."