"I don't remember."
"I don't remember" is just another way of saying "That isn't important to me." Or, worse, it is a way of saying "I don't want to talk about that with you, so I am lying to you instead." It is a coping technique. In any case, it SHUTS DOWN the conversation. "Well, I guess THAT topic of conversation is over. I mean, she doesn't remember, so that's that." How nice for her! And super annoying for me.
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Addressing a problem for discussion to shut it down, or denying that a problem exists, doesn't help anyone, especially in a relationship; it creates annoyance and eventually a pattern of mistrust, as a result of a one-sided relationship where the other is left at a disadvantage and vulnerable.