At Christmas when we were home visiting the ILs, happened to notice as I poured milk into my coffee that it was goat's milk. Forgot to say anything to DH, and it was the end of the day when I remembered. Aside from his wrinkling his nose and proclaiming disgust at it
he had not noticed at all that he'd had goat's milk in his tea and coffee. DS had drank all of his milk without any comments and both MIL and FIL had eaten it on cereal and not noticed the difference at all. MIL looked in the recycling bin and the rinsed jug from the day before was goat's milk, so we had all been drinking it for 3-4 days without noticing it at all
The jugs look almost the same from whichever supermarket she got it and she had just absentmindedly picked them up, none of us had ever tasted it before!