Improper cleaning of your stainless steel is the most common cause. This could include not removing all the dirt from your stainless steel, not thoroughly rinsing your stainless steel after washing, leaving some soap behind, and not thoroughly drying your stainless steel, allowing the iron in your water to react with the oxygen in the air leaving spots behind. In the last instance, it is your water that is rusty not your stainless steel. The best way to avoid rust or corrosion problems is to follow the instructions in the Knife Care Section.
Another cause of rust or corrosion could be that some iron particles have contaminated the surface of your stainless steel by scouring it with steel wool. It is these iron particles from the steel wool that are rusting not your stainless steel. Chlorides, which include a wide range of products from chlorine detergents to salt, can also have a corrosive effect on stainless steel. The addition of molybdenum to all the knives we sell helps our knives resist corrosion to chlorides, but care should still be taken to thoroughly rinse any chlorides from your stainless steel as soon as is conveniently possible.