From an entry called Manners in:
Life Is Meals
A Food Lover's Book Of Days
by James and Kay Salter
The true mark of courtesy is for the host or hostess to casually commit the same mistake as the guest to show that it is perfectly all right. The opposite of this once took place at the White House after lunch when President Calvin Coolidge, a taciturn man, put some milk into his coffee and slowly poured it into his saucer. His guest politely imitated him. Then Coolidge reached down and put the saucer on the floor for his cat.