When Ross Barkley slammed home an equaliser against Manchester United in October 2018, the Chelsea staff and players were, understandably, ecstatic. However, when the Blues' first-team coach Marco Ianni ran along the touchline to celebrate - goading Mourinho as he walked back to his seat - things quickly took an unexpected turn.
It was Chelsea who had dominated the first half at Stamford Bridge, and United were struggling to get out of their own half. Maurizio Sarri's men were extremely well-coached and Mourinho's men were finding it difficult to compete with the London club. With that in mind, then, it was inevitable when the Blues took the lead before the half-time break, with Antonio Rudiger heading home from close range.
David de Gea was in good form and had made a number of smart saves to deny Chelsea, but it felt as though United were going to concede again. Despite the pressure that the Reds were under, it was actually Mourinho's side who grabbed the next goal to level proceedings in the second period. Ashley Young's cross deflected off Jorginho and then Marcus Rashford, before Anthony Martial calmly converted past Kepa Arrizabalaga.
United, to their credit, had made a good start to the second half. The Reds took somewhat of a surprise lead after 73 minutes, with Juan Mata doing extremely well to find Martial on the edge of the penalty area, who curled a shot inside the far post.
The home side - stunned after falling behind in the clash - pushed hard for an equaliser, but it appeared that the Reds would be able to hold on and secure an important three points. That was until the 96th minute, of course.
David Luiz's late header cannoned off the post, with Rudiger heading the rebound straight at De Gea, before substitute Barkley fired home deep into added time.
Chelsea coach Ianni, an Italian who was officially Sarri's second assistant, celebrated the late goal by running across the United bench and clenching his fists close to Mourinho's face. The 40-year-old, who worked with his countryman for two years at Napoli as a technical assistant, quickly made a beeline for his seat after - probably - realising that he had made a mistake.
An incensed Mourinho leapt up and attempted to chase Ianni down the tunnel, which prompted staff, stewards, officials and players to get involved on the touchline. As the incident fizzled out, and the Portuguese coach sat back down, some Chelsea fans started chanting against Mourinho.
In typical fashion, it was the then United boss who managed to enjoy the last laugh. After applauding the travelling Reds supporters, he held up three fingers towards the Chelsea fans - reminding them he won three Premier League titles for the club.
Sarri had immediately accepted that Ianni's behaviour was inappropriate and apologised to Mourinho, before the coach approached Mourinho after full-time. The Chelsea boss said at his press conference after the game that he brought the Italian to the United manager so that he could apologise in person.
Asked about the ruckus after the game, Sarri said: "At the end of the match it was very good, a fantastic match, in the first half in the tactical part we did very well but in the second half we played their match, the physical match which is not for us.
"Sincerely I have not seen [what happened] but I have spoken with Jose [Mourinho] then I have spoken with a member of staff because I think we were on the wrong side so I have dealt with it immediately.
"If I speak to a member of staff and am very hard [with him] I cannot come here and tell everything because then I am not credible with the staff."
Following the match, Mourinho was asked what happened when the goal went in. Somewhat surprisingly, though, he was keen to dismiss the subject, despite his anger in the moment.
"Oh come on, I can tell you that 97 minutes of the game was so good that you have to focus on that," said Mourinho.
"I did not get respect back from Chelsea [fans] but that is not my responsibility. What I did here today I will do in Madrid, in Milan, in Porto, the reaction from the fans is not up to me.
"I am not annoyed with anything. What happened with Sarri's assistant, Sarri was the first one to come to me and say he will resolve it."
Ianni was fined £6,000 by the Football Association after admitting to an improper conduct charge for his overexuberant celebrations, with both clubs, and Mourinho, being reminded of their responsibilities in the aftermath of the incident.
Mourinho actually went on to launch an impassioned defence of the Chelsea coach after he was fined by the FA - insisting that he was well aware about making mistakes and the potential consequences of them.
"I'm not happy that is going too far with the young boy," he said. "I don't think he deserves more than what he got. He apologised to me, I accept his apologies, I think he deserves a second chance, I don't think he deserves or to be sacked.
"His club was strong with him and he went through a situation he recognises he was wrong," he added. "I hope everybody does the same as I did which is not to disturb a career of a young guy which is probably a great guy, is probably a coach of great potential and I'm not happy with it more than that.
"It's the end of story, with me. Let him work, everybody makes mistakes, I made mistakes."
Rudiger, who played an important role in Chelsea's late equaliser, insisted that he wasn't sure what had caused the commotion on the touchline. However, the German - seen in the middle of the melee - suggested that it was caused by 'emotion' and that the situation was quickly resolved.
He said "I don't know [what happened] I just saw the benches standing up and it was like a fight and speaking. It's emotions, it's football. I didn't see everything and it continued on the pitch but that's emotion. It's OK, at the end of the day we shake hands.
"The game was still going on. It's emotion but everything has calmed down now so everyone can go home."
Mourinho and United had endured a difficult start to that season. With just seven wins in the first 17 Premier League games, the Portuguese coach - who had won the Europa League and FA Cup with United the season before - was sacked by the club in December 2018.
He was at the helm when the Reds won their last trophy, and left the club with a win ratio of 58.3 per cent in all competitions. That, while admittedly not a measure of success, surpassed the record of his predecessors Louis van Gaal and David Moyes.
The 59-year-old, who is currently in charge of Roma, previously claimed he was not given enough time by United after they sacked him in 2018, but did insist that he respected the club's decision. Mourinho insisted that it was a unique project that required more work than his previous clubs.
As others that followed him have discovered, he was certainly not wrong.
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