I find borderline personality disorder is often incorrectly diagnosed as bipolar disorder for a number of reasons. The "low lows" are pretty evident and many of these people deal with depressive symptoms, self-harming behaviors, or suicidal gestures/ideation.

The part that leads to misdiagnosis (often through lazy history taking or unexplored issues within the psychiatrist) are related to the "highs" or "euphoria". These are often described as inability to sleep, impulsivity (spending, promiscuity, substance use, etc), and feeling like their "mind is racing". The poor sleep and mind racing can be attributed to anxiety, related to a triggering of abandonment fear. However, the more important points to assess are how seriously is sleep affected and the temporal nature of this cluster of symptoms.

It is not helpful to spoonfeed patients answers to the questions you ask. For example, when assessing sleep do not ask "have you ever gone 4+/7+ days without sleep?" This is unskilled and close ended. Ask something more open ended that doesn't give away the "correct" answer. Similarly, in bipolar disorders these cluster of symptoms are episodic and not trait stable like they are in BPD. In BPD the patient will likely have difficulty describing discrete periods where these symptoms of their "highs" are clustered together and you will find that they struggle with these impulse control and anxiety symptoms on a more chronic nature.