Jude Bellingham broke a second little-known rule before Real Madrid's 3-1 play-off round win over Manchester City - but he is not the only player to do so.
Kylian Mbappe scored a stunning hat-trick for Real, before Nico scored a consolation in second half stoppage time. Los Blancos won 6-3 on aggregate.
Real went into the second leg clash at the Bernabeu 3-2 up after an excellent victory at the Etihad Stadium last week.
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Bellingham scored the winner in second half injury time on his return to England, but has been involved in major controversy in the days since.
He was sent off during Real's 1-1 La Liga draw with Osasuna on Saturday, with referee Jose Luis Munuera Montero claiming in his match report that the midfielder said 'f**k you' to him.
Bellingham, however, told reporters that he was saying 'f**k off' to himself. There is an important distinction to be made between the two, as verbally abusing a match official in such a manner can lead to a suspension of between four and 12 matches.
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On Wednesday, the Spanish FA (RFEF) handed him a two-match La Liga ban for 'attitudes of contempt or lack of consideration towards referees'.
He thus avoided a more serious suspension, with the sanction also meaning he was available for the Champions League clash against City.
The match was one of the few occasions where Bellingham avoided technically breaking a little-known rule - although he did receive a yellow card which will see him suspended for the first leg of Real's last 16 clash.
UEFA does not have specific regulations relating to the cutting of holes in socks - a practice Bellingham and a number of other international stars, including Kyle Walker and Conor Gallagher, also stick to.
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The main reason behind the practice is to protect muscles in the calf, as tight-fitting socks can inadvertently apply heavy pressure to the muscles and restrict their movement.
They can also affect bloodflow and circulation through the legs, particularly those with bulging muscles. The holes can, therefore, improve breathability and range of movement, as well as restricting circulation issues.
However, in every La Liga and FIFA match that Bellingham plays in, he is technically breaking the rule as neither organisation permits hole-cutting in socks.
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The restriction also applies to Gallagher, who signed for Atletico Madrid last summer.
Gareth Bale also broke the rule when he played for Real Madrid.
The Premier League does not have a written regulation surrounding the practice, with Noni Madueke, Alejandro Garnacho and Bruno Guimaraes among those who regularly cut holes in their socks.
The infraction, however, is not punished by La Liga. Spanish outlet AS claim that the rule is not strictly enforced by La Liga as they are aware that there could be 'medical chaos' in forcing players to wear clear socks - meaning they 'turn a blind eye'.
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UEFA's regulations, meanwhile, state: "Tape, strapping and/or any other material used over playing socks to hold socks in place must be transparent, or of the same colour as the part of sock to which it is applied, and must be free of team and manufacturer identification and decorative elements."
Topics: Jude Bellingham, La Liga, Champions League, Manchester City
Ryan Smart