This happened on the 28 bus in Boston. If you look at a map of the route and a map of murders in the city you'll notice they match up perfectly.

Last August, a woman got onto the bus and refused to pay her fare. Some sort of argument started between her and the bus driver, so another passenger started recording a video on their cellphone that they then uploaded to the WorldStarKnockouts. Their Facebook page posted it, and by the next day it had been shared over 13,000 times. The Facebook video is deleted now and their website gives me some malware warning (and it's probably deleted from there too), but it looks like someone copied it to LiveLeak, so you can watch it there.

I'd never be able to do a better writeup than this Turtleboy Sports article titled "Woman Who Threw Drink At, Got Beat Up By MBTA Bus Driver In Front Of Child Deserved an Ass Beating More Than Any Person Ever", but if you want a tl;dr she throws a drink at the bus driver, gets off the bus, the bus driver runs after her, she tries using her child as a human shield, and the driver knocks her down and starts punching her until someone else pulls her off, at which point the two of them continue screaming and swearing at each other.

It obviously caught the attention of local news, and the MBTA released a statement saying:

Transit Police are working to identify the woman who assaulted the bus operator. The woman was upset because the bus operator had asked her to pay the bus fare. It's unclear why the woman felt she didn't have to pay. The bus operator is suspended while a full investigation takes place. While it’s very clear that the passenger was the instigator, it’s incumbent upon the bus operator to demonstrate restraint and allow police to handle such a matter. Two wrongs do not make a right.

They also described the bus driver as "a very good employee" and confirmed that she had driven there for over 13 years.

A week or two later, a newspaper got an interview with the woman the threw the drink and got beaten up, which is now behind a paywall but was a predictable sob story about how the driver started attacking her out of nowhere, she had to take the bus she couldn't pay for because she was something like two weeks pregnant (I'm sure...), and now she and her kids are traumatized for life and too scared to go out in public (coming from the person who not only started the fight, but then tried using her daughter as a human shield) so she was laid off from her job and is suing the MBTA.

Turtleboy Sports has another great writeup about that, which matches up her version of events with video clips and reveals that her "political fundraising" job of 12 years was actually being a prostitute, titled "Update: Mom Beat Up By Bus Driver For Throwing Drink Is A Pregnant Retired Prostitute, Blames Driver For Not Letting Her Ride For Free".