>>> How do you accidentally transfer contaminated product into fuel?
Probably remarkably easily. (It's little different to accidentally mis-fuelling a car, which happens hundreds of times every day. I've done it myself!).
Contractors have pumps which remove contaminants from the bottom of fuel tanks at petrol stations and put them into storage tanks in their own vehicles. Then they pump them out again at an authorised site for disposing of them.
All that's needed for things to go wrong is for a contractor's employee to press the 'Pump Out' button at a filling station, instead of the 'Pump In' one, and all of the contaminants that were already stored in his vehicle (collected from lots of other filling stations) will end up in the filling station's fuel tank.
At least it was only one filling station (and a small number of cars) that had problems, unlike back in 2007 when hundreds of filling stations (and possibly around 10,000 motorists) suffered from problems with contaminated fuel:
https://en.wikipedia.org/wiki/2007_United_Kingdom_petrol_contamination