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.

Off the top of my ancient head:

Trump is forcing this dirty, costly coal plant to stay open - The Washington Post

Obsessing about clients outside of work hours.

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

General Introduction to Boundaries from Panahi Counseling:

What are some reasons why some men choose to live alone instead of getting married?

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

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.

What are the pros of using stainless steel wire mesh in gas filtration?

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

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

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

New Study Finds No Evidence of Tension in Hubble Constant - Sci.News

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.