In an electronics store in Amstetten, in German. Mistake (and irony) realized after the fact.
Friendly Cashier: You're from Austria, are you?
Me: No I'm not. I'm from America. I'm on my way to Linz.
Friendly Cashier: Wow, your German is quite good.
Me: Thank you.
Friendly Cashier: Well, I hope you have a nice vacation here in Austria!
Me: You too!
Friendly Cashier: [silent retraction of compliment]