Are the items dishwasher safe?
Inexpensive, pliable, and/or thin-walled plastics are usually indications the item is not dishwasher safe. It is not recommended that such items be washed in a dishwasher. Always refer to the manufacturer's washing recommendations to see if an item is dishwasher safe.
Was a heated dry cycle used?
Heated-dry cycles can cause temperatures much higher than even some dishwasher safe plastics may not be able to tolerate. Consider using the air-dry option or loading items in uppermost racks and away from the heating element.
Was a high-temperature cycle used?
Some cycle and dry options can drive final rinse temperatures to over 150F, which can cause some plastics to warp or misshapen.
Were the items loaded correctly?
Don't force plastic items into crowded areas or allow heavy objects to rest on them. As the plastic is heated, it can become softer and can be deformed by the forces acting on it.