He lost just two of his 38 fights, recording 14 KOs in the process.
But Flanagan is unlikely to have celebrated any of his wins more than Haaland's timely strike.
The incident happened an hour into City's 2-1 win at Bramall Lane on Sunday.
Haaland had already missed a penalty against the Blades as City struggled to find a way through.
But the Norwegian hitman finally got off the mark with a well-taken close-range header following a cross from Jack Grealish.
And as he ran off to celebrate, Flanagan jumped from the stands onto the pitch.
He then launched himself onto Haaland's back as he attempted to join in City's celebrations.
The forward did not appear to be taken aback by the incident, despite Flanagan appearing to kiss his neck.
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