What problems happen when a latex paint is applied over a oil based enamel?

What problems happen when a latex paint is applied over a oil based enamel?
I checked out the Lowes.com web site and this is what they had to say.

Can I apply oil-based paints over latex paints, or latex paints over oil-based paints?
The most important thing to know is the condition of the surface itself. Regardless of whether the base coat is oil or latex, you must dull glossy paint by sanding or deglossing with a chemical deglosser. If you don't dull glossy paint, the second coat will streak and not stick to the base coat when drying.

Click on the below link and it will bring you to Lowe's

Tips on Painting. (http://www.lowes.com/lowes/lkn?action=pg&p=HomeDecor/paintProjCtr_tips.html&rn=none)