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

Serious disappointment when the client cancels a session.

Off the top of my ancient head:

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

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General Introduction to Boundaries from Panahi Counseling:

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

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

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

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

Obsessing about clients outside of work hours.

A 35-year-old woman suffered a stroke on an international flight. A Houston doctor believes he knows why. - KHOU

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

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

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

'Classmates threw Snickers at me to test my peanut anaphylaxis' - BBC

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.