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

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

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

Serious disappointment when the client cancels a session.

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Obsessing about clients outside of work hours.

Off the top of my ancient head:

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

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

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

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

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

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.