This may occur if:
When printing an image overlaying the white toner with the cyan, magenta, yellow, or black toner
When using transparent paper
When the cyan, magenta, yellow, or black toner overlaying with the white toner is thick
When the cyan, magenta, yellow, or black toner is used at the trailing edge of an image using the white toner
To adjust the following settings, pre-register the type of paper in use as a custom paper. For details about registering custom papers, see "Registering a Custom Paper", Paper Settings.