While at Dongdaemun this weekend, I asked a saleswoman IN KOREAN, "세-트예요?" ("Is that a set?") I was trying to pronounce "set" with Korean parum, so I think the word came out sounding loud and awkward.
The woman replied with a patronizing gleam in her eye "세-트않이에요," mimicking my choppy and hesitant Korean accent. I know that I tend to be overly sensitive, but I'm pretty sure the woman was mocking me. She could have meant it in a more good natured way, but there was definitely some mocking going on.
Unless you know a person fairly well, it's so not cool to make fun of a person's accent, especially when she is still learning a language. I know that some teachers promptly correct the English pronunciation of their Korean students. I try to avoid doing this because I figure that it's more important for students to get the words out and speak, rather than obsess over proper pronunciation -- something I always struggle with when I attempt to speak Korean.