I think I have found a contradiction in the books:
In book 6 we are told (twice) of the exceptions to Gamps Law of Elemental Transfiguration, one of them being Food. However in book 4, 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 Book 6, we see the “Aguamenti” Charm to create water. We also (4) see Olivander (when examining Harry’s wand) “made a fountain of wine shoot out of it”.
Dumbledore also manages to conjure “a dusty bottle and five glasses appear in midair. The bottle tipped and apoured a generous measure of honey-coloured liquid into each of the glasses… “Madam Rosmertas finest oak-matured mead”” (Although it could be he somehow summoned the mead from somewhere else, because he himself said it is Madam Rosmertas finest…)
In the Pensieve as well, in book 6, we see a younger Dumbledore having a meeting with “Mrs Cole”, and “her eyes fell upon a bottle of gin and two glasses that had certainly not been present a few seconds before”? (which sounds like Dumbledore conjured them)