I never realized how funny the phrase "have a nice day" (spoken by cashiers while you're checking out) was until I went to Europe -- all my non-american friends thought what was a strange custom. What does having a nice day have to do with completing the transaction?? Sure it's a friendly thing to say, but the customary time and place (at the cash register) does seem a bit strange when you think about it...
And on the flip side of the "automated responses" thing, sometimes when it seems the responses are going to be automated when a salesperson greets me, I'll say something strange just to keep them on their toes. Like you said, half the time they don't even notice
