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Joao Cancelo has the chance to become the latest player to haunt his parent club when Bayern Munich face Manchester City in the Champions League quarter-finals on Thursday 11 April at Ethiad Stadium.
Unlike in the Premier League, Uefa competitions allow players to play against their permanent employers.
BBC Sport looks at some previous times players have made their parent clubs pay.
LuaLua goal gets Premier League rule changed
Until 2003 Premier League clubs could not sign players on loan from other teams in the division. That rule needed tweaking again because of Lomana LuaLua.
Portsmouth signed the DR Congo winger on a three-month loan deal from Newcastle in February 2004 – the first season such moves were permitted.
Four weeks later, on his first start for Portsmouth, he netted an 89th-minute equaliser against his parent club.
“I was a bit surprised I was allowed to play, it is not usual in these sort of deals,” said LuaLua – who celebrated in front of the Magpies fans.
Newcastle boss Bobby Robson, whose side ended up missing out on the Champions League that season by four points, admitted: “He was the last player we wanted to score against us. One of our players has taken a point off us.”
LuaLua joined Pompey permanently at the end of the season – and the Premier League decided loan players could not face their parent clubs any more.
Nielsen scores and gets keeper sent off
David Nielsen joined Norwich from Division One (now Championship) rivals Wimbledon in a loan move in December 2001.
In his second game for the Canaries, he faced the Dons. The striker opened the scoring with a diving header and then won a penalty which Iwan Roberts scored.
Dons keeper Kelvin Davis was so angry with a player who was technically still his team-mate that he threw the ball at the Dane and was sent off.
Norwich won 2-1, Nielsen joined them permanently after scoring five goals in five games and they beat the Dons to a play-off place.
Courtois frustrates Chelsea after £5m dispute
This one was so controversial the authorities ended up getting involved.
When Chelsea loaned Thibaut Courtois to Atletico Madrid for a third season in a row, they said if the sides met in the Champions League that the goalkeeper could only play if Atletico paid them a reported £2.5m fee per game.
The sides were indeed drawn against each other in the 2013-14 semi-finals, but Atletico said they could not afford to pay the fee.
They went to Uefa, who said such a deal that “might function in such a way as to influence who a club fields in a match is null, void and unenforceable” and Courtois could play without £5m being shelled over.
The Belgian kept a clean sheet in a goalless first leg and made four saves at Stamford Bridge as his side progressed 3-1 on aggregate.
He was one of the few players in this article to go on to have a career with his parent club, spending the next four seasons as Chelsea’s number one before joining Real Madrid.
Coutinho nets twice – and then Bayern don’t sign him
It is probably fair to say Barcelona wish they had never bothered signing Philippe Coutinho from Liverpool in 2018 when they made him the third most expensive signing ever – for a fee of up to £142m.
He underwhelmed in his three and a half seasons in the Barcelona team – and had a hand in one of the most embarrassing nights in their history.
The Brazilian forward joined Bayern Munich on loan in 2019-20 – and scored twice and assisted one in their one-leg 8-2 Champions League quarter-final win over Barca in Lisbon.
Despite that, Bayern declined the chance to sign Coutinho for 120m euros and two years and just five Barcelona goals later he joined Aston Villa for 100m euros less.
Two Morientes goals knock Real out
Fernando Morientes won the Champions League three times in six full seasons at Real Madrid – but also helped to knock them out.
In 2003-04 he joined Monaco on a season-long loan and netted in both legs of their quarter-final thriller against Real – which they won on away goals after a 5-5 aggregate score.
“It was very difficult for me. In front of me were all my friends, my old team-mates, my big friends Raul, Figo, Zidane. You try to be professional in this moment,” he said years later.
The Spanish striker ended up winning the Champions League Golden Boot that season with nine goals, as his temporary side lost the final to Porto.
Morientes was back at Real at the start of the next season, but England striker Michael Owen kept him on the bench and he joined Liverpool six months later.
Coman comes back to haunt Juventus
Parent clubs, former clubs, it doesn’t matter – Kingsley Coman loves scoring against them in the Champions League.
The France winger, whose sides have won a league title in each of his first 10 seasons as a professional footballer, joined Bayern Munich on loan from Juventus in 2015-16. He had netted just once for the Italian club in a year.
And he made his parent club pay in the Champions League last 16, setting Thomas Muller up in injury time to send the game to extra time and then netting in the 110th minute as his loan side won 6-4 on aggregate.
He went on to join Bayern permanently and in the 2020 Champions League final netted the only goal against his first club, Paris St-Germain.
Vertonghen denies Ajax title
It is always good to send a young player out on loan to give them experience… except if it costs you the league title.
Ajax probably did not think too much of it when they loaned young Belgian defender Jan Vertonghen to struggling RKC Waalwijk in January 2007.
But on 8 April, with four games left of the season, he scored in Waalwijk’s 2-2 draw with Ajax. Three weeks later Ajax lost the title on goal difference to PSV Eindhoven.
Without that goal, Ajax would have won the title. To make matters worse Waalwijk were relegated anyway.
Ajax and Vertonghen would have to wait another four years to win the title together.
