I habitually thank MBTA operators whenever I pass them on my way off a train or bus.  I say, “thank you,” and if they respond it’s generally with “you’re welcome” or “have a nice day.”

I’ve done this every day for three years in Boston, and before that every day for probably seven years on Colorado’s buses.  Until today, I’ve never given the matter any thought at all; those are just the good manners one exhibits in polite society.

Tonight, when I stepped off the train, the exchange went like this (all quite clearly enunciated, so I’m sure I heard correctly):

Me: Thank you.
Motorperson: (with extreme annoyance) Thank yourself!

Is this like how “bad” was used to mean “good” for a while in the 1990s?  Did I just insult someone’s mother inadvertently?