This was reported by the likes of Robert Peston and Laura Keunssberg, as well as the less credible Guido Fawkes. It also quickly made it into the Mail, the Sun and the Express.
Video quickly emerged proving it didn't exactly 'go down' as had been claimed - a protester was gesturing, and the SPAD walked into his (unclenched) hand.
Keunssberg and Peston, eventually acknowledged they were spreading false information and apologised:
Happy to apologiSe for earlier confusion about the punch that wasn’t a punch outside Leeds General - 2 sources suggested it had happened but clear from video that was wrong— Laura Kuenssberg (@bbclaurak) December 9, 2019
But note that lines: "2 sources suggested" and "I was told by senior Tories" - who the Hell fed them the misinformation? And why are they protecting them?It is completely clear from video footage that @MattHancock's adviser was not whacked by a protestor, as I was told by senior Tories, but that he inadvertently walked into a protestor's hand. I apologise for getting this wrong.— Robert Peston (@Peston) December 9, 2019
The real story isn't that a protestor didn't punch someone. The real story is that 'senior Tories' are feeding lies via respected journalists and those journalists are diligently repeating those lies. And - even when they are caught - the journalists are so cravenly, sycophantically aligned with the 'senior Tories' they will cover up who is trying to feed falsehoods into the election campaign.