You can use any interaction that caused you discomfort, as long as there is a beginning and a clear ending and the event does not last more than a few minutes.
An ideal situation is often no longer than - He said XX, I said YY. Getting the full context is not as important as trying to accurately capture what was said. Don't worry about choosing the perfect example of a difficult conversation, or you will most likely not complete the exercise.
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The key component of this description should be an exact set of quotes. What did the other person say that was most difficult or distressing to hear? It helps to write: He/She said: "XX."
This should be an emotional response not an analytical one. "I felt she hated me," would be appropriate. "I thought that perhaps she might have said that in a more positive way," would not.