Spain's Sergio Ramos should have been allowed to retake his penalty against Croatia on Tuesday after goalkeeper Danijel Subasic came off his line early, UEFA's head of referees said on Friday.
Pierluigi Collina acknowledged it was wrong of referee Bjorn Kuipers to let Subasic advance so far to save Ramos' penalty.
The save kept the score 1-1 before Croatia went on to win 2-1, topping the group and sending Spain to a round-of-16 match against Italy.
"Unfortunately, that goalkeeper moved forward, it was not spotted by the referee's team," Collina said. "It was a mistake in a match refereed well."
Ahead of the knockout rounds, where penalties are likely to come into play, Collina said he stressed a strict interpretation of the rules.
"Of course, referees should pay attention that the penalty kicks are taken correctly," Collina, who officiated the 2002 World Cup final, said at a briefing for the remaining 12 referees, including Kuipers.
"It will be retaken and, as the laws of the game say, the goalkeeper will be cautioned."
Subasic's save created confusion about UEFA's current interpretation of Law 14 regarding the penalty kick, which requires goalkeepers to stay on their line until the ball is touched.
"It was a mistake, unfortunately it happens," Collina said. "It is not a trend. It was one single incident where the goalkeeper moved before the penalty kick was taken."
Collina also reminded that penalty takers can be punished twice for breaking the rules.
"Whenever the penalty taker makes an illegal feinting, the penalty is not retaken and the player is cautioned," the Italian official said.
Players from the 16 teams remaining at Euro 2016 will not get a special reminder of the rules on penalty kicks.
"Everyone should know what he is allowed to do or not," Collina said. "We cannot start recommending to each player to do something or not do something. It is not the work of the referee or the [match] delegate.
"We trust in players."