slip of the tongue when someone says something that they did not intend to say I called her new boyfriend by her previous boyfriend's name - it was just a slip of the tongue.

Sandy boy is right about "slip of the tongue. "It's on the tip of my tongue" describes the situation when you are about to say something and you forget what it was. It's not something that you don't know at all or have completely forgotten. It's the opposite, you know that you know it, but you just can't recall it at that moment. Then there is "it slipped my mind", which I think is used more for forgetting to do something, like calling someone back, picking something up, etc.

"slip of the tongue". when someone says something that they did not intend to say I called her new boyfriend by her previous boyfriend's name - it was just a slip of the tongue.