What items should I NOT wash in my dishwasher?

Do not wash iron (will rust), pewter (will tarnish), tin (can rust), or wood items (can warp, crack or lose finish). Also avoid non-dishware items such as electronic air cleaner filters, furnace filters, and paint brushes. For plastics, check that they are marked 'DISHWASHER SAFE' before washing, as damage to the dishwasher and staining may result from inappropriate items.