h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The only places where Mario and Luigi were ever referred to as plumbers were loosely-licensed bits of fiction such as the comic book and the film. Nowhere in any of the games are Mario or Luigi ever treated as a plumber, nor do they ever "lay the smack down" with a plunger or drain snake. The film has, gladly, gone the way of the virtual boy.

Maybe the games themselves don't call Mario and Luigi plumbers (the games do hint at it though, as they travel through pipes) but if you look through most (all?) Mario game instruction manuals, you'll find that they have always been plumbers, long before the toilet fodder movie came out.