Today I learned that aluminium foil in the dishwasher can help against flash rust. The idea is that the small ball of aluminium foil hangs around in the cutlery section of your dishwasher. When small amounts of rust appear during the washing process (due to being washed from something within the dishwasher), the aluminium itself will oxide and due to the high temperatures and bind the rust from the water to itself.

I learned about that while talking to a friend and thought some trash somehow made it into the dishwasher. After he explained it was intentional, I did some basic research and came across some articles (German) that supported the idea. There is even an English speaking blog article I found after a first quick research on the topic in English.