Might help to think of boundaries as sort of an extension of oneself, including the unconscious mind.

Hypothetical examples of how that can play out--

"I need the approval of this person so badly, that I will sacrifice myself. I will allow them to take, take, take, until I have no more to give." (also revictimization) This is sort of boundryless person. The person asserts no boundaries and others who are predispositioned will take advantage whether intentional or not. A therapist can sight of the t-relationship by being swept away by their own issues here.

"I have no worth, so I will give, give, give, take care of others' needs, while ignoring my needs" This leads to revictimization as well....This could be a therapist responding to every whim of a needy client. Or, a victim of child abuse getting revictimized...

"I need to be a savior" A therapist with this persona is really prone to serious boundary violations as they can lose sight of therapy altogether. For example, this could be the type of therapist who invites the client to his/her home as nothing will stand in the way of saving the client

"I am so absorbed/anxious/worried about my own needs getting met, that I do not even see the other person's needs" This could be a person contacting the therapist way too much though innocently. The person turns into the perpetrator

"If I let someone get to know me, they will reject me. I can't let myself be vulnerable lest be hurt" This could drive a therapist to the profession so that they can have relationships with others (clients) while staying safe and distanced

"My needs are more important than others' needs. Their needs don't matter". This is someone who may purposely take from others, objectify, and violate. Abuse can be thought of as a violation of another's boundaries.

This is one of the main reasons why it is so important for a therapist to have gone through depth therapy themselves!

Healthy boundary:

"I take care of my own needs and want to be there for my clients as well, while maintaining professionalism. I know myself well enough to know I tend to be a people pleaser, so I will limit emails to x per week, etc"

I think Puzzlebug said it elsewhere-poor boundaries is a sign of an unsafe therapist. It could be innocent, a result of that person's own upbringing. I wouldn't put misjudgments in this same category though as everyone makes mistakes.

A therapist should be keenly aware of these things--but in themselves as much as in their clients. Looking at those examples, you might see how people who are victims of abuse can be revictimized by not asserting boundaries. One main reason why it is the therapist's job to manage boundaries rather than blaming a client when things go wrong.

Boundaries are much, much more than rules. They reflect your character. They are very important!