Banjo fittings are ubiquitous on engines (there are a lot of them around, too), usually in the fuel system, but the fresh water cooling circuit often has a few as well.
And each of them has a small copper washer each side of the fitting that acts as a seal.
Here’s the thing: the washers are not reusable…but most people do, including many professional mechanics who should know better.
Which is why a lot of engines are plagued with leaks.
Thanks to my friend Wilson, who warned me about this when we bought the J/109, so I had the right washer to hand when the engine started pissing diesel fuel.