Magento can be configured to display a notice when your store is in demo mode so that your customers know that their orders will not be processed.

In order to enable/disable the Demo Store Notice in Magento, you need to log into the Magento administration area and navigate to the System tab > Configuration. Select the Design section, click HTML Head  and you will see a Yes/No drop-down menu for Display Demo Store Notice.

Enable/disable the demo store notice and click Save Config to save the change.