When the 3 visitors show up, Avraham indicates he will get them bread. He then runs off and tells his wife not just what to make but what recipe to use (Gen 18,6):
"וַיְמַהֵר אַבְרָהָם הָאֹהֱלָה אֶל־שָׂרָה וַיֹּאמֶר מַהֲרִי שְׁלֹשׁ סְאִים קֶמַח סֹלֶת לוּשִׁי וַעֲשִׂי עֻגוֹת׃ "
Abraham hastened into the tent to Sarah, and said, “Quick, three seahs of choice flour! Knead and make cakes!”
Did she not know how to make bread? Would she normally use a non-flour product or the wrong amount? He says "asi ugot" (make cakes) but he doesn't trust her to know how? Couldn't he just have told her to make food (in general or in specific) and let her collect the ingredients?
The commentators discuss why "cakes" (for speed's sake) and why he tells her to knead the bread (to avoid it's turning into chametz) but I haven't seen anyone discuss why he needed to tell her the amount, or, more precisely, why he assumed she wouldn't know the right amount.