The emulsion (mix of fat and liquid) doesnt hold.
you might avoid this by using cold butter when creating your sauce.
Chef Mario Batali shares this tip among his list of the ten most important pasta cooking tips.
This is true for other emulsion sauces as well,Fine Cookingpoints out, such as hollandaise sauce.
The 10 Most Important Pasta Cooking Tips| Mario Batali
Photo byK.