What are some signs that a therapist may have poor boundaries with their clients?

Serious disappointment when the client cancels a session.

Session-expressed curiosities about client details not relevant to the therapy.

Off the top of my ancient head:

Why won't my mom let me come home if I'm homeless?

General Introduction to Boundaries from Panahi Counseling:

Routinely going over the time limit with certain patients, compromising the time for the next client.

Sense of competition with persons who are important in the client’s life.

Why don’t people want the American Dream anymore - marriage, kids, a dog, and the white picket fence?

These items can happen fleetingly, briefly, in any therapy, but if they’re frequent, it’s definitely time for the therapist to get some good, solid supervision/consultation.

Obsessing about clients outside of work hours.

Failing to mention the client in supervision/consultation, out of fear the supervisor/consultant will advise return to ordinary healthy boundaries.

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Disclosing feelings, fantasies, and experiences to the client in ways not related to the work the client is engaged in.

Frequent phoning or texting of clients to “check up on them and make sure they’re OK.”

Struggling with fantasies of deeper connections with clients, whether sexual or parental or other intense or intimate relationships beyond psychotherapy.

Gun owners, imagine if an attacker comes to your home and takes your gun to use against you before you had the chance to pick it up. Would you regret owning a gun?

Eager anticipation (or anxious anticipation) of the next session in ways that distract.