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

Serious disappointment when the client cancels a session.

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

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

What were Sauron's powers in The Lord of the Rings (LOTR)? Did he have any magic or anything like that?

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

General Introduction to Boundaries from Panahi Counseling:

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

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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.

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

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

Why does my vagina always itch so badly after my periods?

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

Obsessing about clients outside of work hours.

Off the top of my ancient head:

Why do most people care so much about what others think? Are they afraid of society norms?

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