It's not to excuse the failure by Fein's factory - as what you describe IS a manufacturing defect, but Fein are German and the majority of these machines will be fitted with round pin Schuko socket on the front and a matching plug on the mains lead.
Schuko fittings are not polarised so the assemblers in the factory won't be required to wire L and N a particular way round. In fact, inside the machine you may find it's all wired in the same black wire.
It's an interesting intellectual question to wonder if the defect in yours is common and if after all this time, Fein could be pushed into a mass safety recall on all UK product shipped to date.
Ouch !
They're a decent enough company, but I bet they wouldn't.
As a contrast, there is one British firm that discovered a safety issue in a best selling electrical testing product it had been manufacturing for years, and they did trigger a mass product recall and replacement. The problem with their product was probably no worse than the issue described by the OP. Martindale are a stand-up company if you ever need an electrical tester and they properly understood "reputation management" on the day they made that decision.
@Gui
Just use the machine plugged into a circuit that has RCD protection on it. Job done, but for curiousity, use a test meter or battery and bulb and check if the earth is wired through from the plug to the power take off. The vac itself will be double insulated so except to provide earth to the socket, it might not even have an earth !
To fix it, the error will either be where the power lead comes into the machine. This is (probably) the easiest place to open and swap the brown and blue wires over. Or the brown and blue could be swapped at the back of the front panel power outlet. If a service tech was fixing the machine, they would have to check both.
For anyone else: use one of those Martindale socket testers to check if the PTO on your own shop vac is wired through correctly, just like you would use it to check the electrical sockets throughout your house. First thing you do when you buy one (a house), yes ?
