United travelled to the San Siro in the quarter-finals of the competition in 1999 on their way to winning a famous Treble.
The Red Devils took a 2-0 lead from the first leg to Italy, courtesy of a Dwight Yorke brace.
Inter, however, did possess the best striker in the world in Ronaldo, while Diego Simeone was ready for a fierce midfield battle with Keane.
But Keane did not care too much for the duo, walking straight past them as they adjusted their socks while the players were exchanging handshakes.
The reaction of Ronaldo and Simeone remains priceless to this day, while Keane is still pretty terrifying.
A late goal from Paul Scholes, meanwhile, saw United draw 1-1 with Inter, and we all know what they went on to achieve.
CLASSIC: When Roy Keane mugged off Diego Simeone & Ronaldo. No nonsense. 😂👏 pic.twitter.com/I2Hf0xYtV6
— SPORF (@Sporf) December 4, 2016
These players knew how to make their mark.
This will do your head in.
Featuring plenty of Razor Ruddock.
One of the World Cup’s most infamous matches.
Dean Ashton’s career was cruelly cut short.
Fair play to the Brazil fan dressed as a sticker.
This is outstanding.
Five penalties were given, but only three scored.
His free-kicks alone were worth the entrance fee.
Juan Carlos Valeron was a true footballing artist.