When you are using a dishwasher, some things should never get into the machine. It will be best if you never put wooden or bone china utensils inside the machine. The same goes for cast iron pans, as warm water can ruin the top layer of the utensils. Never put copper or brass utensils inside the machine as the detergent and dampness can destroy its colors. You should also re-check that a plastic item is dishwasher safe before putting it inside. Washing your knives can be tempting, but dishwashers can dull them down. Also, never put painted or crystal ware into a dishwasher as heat is detrimental to them.