In Harry Potter and the Goblet of Fire, when the kids and Arthur get to The Burrow from Privet Drive and Molly is cooking and raging about Fred and George's shenanigans, the following occurs:
Mrs. Weasley slammed a large copper saucepan down on the kitchen table and began to wave her wand around inside it. A creamy sauce poured from the wand tip as she stirred.
In Deathly Hallows, Hermione says that this is not possible:
“Your mother can’t produce food out of thin air,” said Hermione. “no one can. Food is the first of the five Principal Exceptions to Gamp’s Law of Elemental Transfigura—”
“Oh, speak English, can’t you?” Ron said, prising a fish out from between his teeth.
“It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some—”
So how was Molly causing sauce to pour from her wand, seemingly out of thin air?