The 'Ikale Tahi lost the plot in the last 20 minutes, giving away a 15-11 lead, losing 25-15 to Romania in Bucharest on Sunday morning (NZ Time).
It was a disappointing result for what should have been a sure win for the islanders, who only 12 months ago ran in seven tries to beat the Romanians 45-24 at the Rugby World Cup in France.
Head coach Tevita Tu'ifua said it was a disappointing finish in their first of three matches of their European tour.
Leading 15-11, the Sione Havili Talitui led side succumbed to basic mistakes, giving away critical penalties as the hosts took their opportunities and piled up 14 points to claim the win.
A knock-on from a defensive lineout with 19 minutes to go saw winger Tevita Manumua dot the ball down close to the corner flag, with the Tongan-born player just managing to do that under pressure.
That gave the hosts a slender 16-15 lead.
The Tongans then had two sure chances to reclaim the lead. They opted for a kick-out from a penalty inside the Romania 22, and from the ensuing drive were awarded another penalty closer to the line.
They went for the quick tap and drive but were penalised for holding the ball on the ground as the Romanians defended their line.
Then five minutes later, they had another opportunity but lost the ball forward close to the line.
Three successful penalty kicks gave the Romanians victory in the end.
Tu'ifua said it was not the start they wanted.
"We gave that away as we made too many mistakes," the former 'Ikale Tahi said.
"But we will learn and move on to the next one."
The Tongans are building for the 2027 Rugby World Cup qualifiers and a number of young players are being given game time.
Flying Fijians get Europe players back
Meanwhile, the Flying Fijians have named several experienced Europe players in their line-up to meet Wales in Cardiff on Monday morning (NZ Time).
Captain Waisea Nayacalevu, centre Semi Radradra, Josua Tuisova, and Jiuta Wainiqolo have all been named in the starting XV.
The players missed the loss to Scotland because of club commitments.
The Flying Fijians will face Wales at 2:40am on Monday morning at the Principality Stadium in Cardiff.