Yes I do have proof actually, it happened to my R31 a couple of years ago... It conked out on the way to work and after I towed it to work and pulled the fuel tank and pump out (worked as a mechanic at the time - handy) the pump was covered in the same brown slime that was sitting in the bottom of the tank.
It wasnt rust either, the tank was spotless after I drained and cleaned it.
The fuel filter is after the pump not before it, the pump is exposed to whatever goes into your tank. The filter is there to stop whatever gets through the pump and lines from getting into the fuel rail and causing a blockage.
You would be very surprised at what is in most people's fuel tanks, especially if they use crap quality fuel... Even the best fuel stations can get contaminated fuel reserves, if the truck that transported it wasnt cleaned properly or the huge underground tanks are contaminated it all ends up going into some poor bastards car eventually.