After the carpet is cleaned, the cleaning technician should place foiled-covered-cardboard or plastic squares under the legs of chairs. Heavier furniture like filing-cabinet should have a styrofoam or wooden block placed under them so that the carpet can dry and to prevent staining problems. The example below shows how a chair with an iron tip can leave a rust stain.
The picture below shows the culprit
The iron tip reacts with oxygen in the presence of water and hence makes rust.