Years ago, while dining out with my brother the meal was served with a small cup of horseradish for use with the prime rib. He thought it was sour cream for the baked potato... and also ended up with a priceless look on his face after the first bite.
Oh now that would have been bad! At least butter just had a funny taste...not a lasting after effect!
Same thing happened to me in a dark restaurant. Hubby still laughs about it. They also used a whipped horse radish instead of the raw stuff most places use. Looked just like whipped butter.
There seems to be a lot of this going around. On my aunt's first visit to a Japanese restaurant, she thought the little cup of green stuff was guacamole and downed it all in one bite. She learned real quick that you just don't DO that with wasabi (horseradish)!!