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

Off the top of my ancient head:

Serious disappointment when the client cancels a session.

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

Why does an older married man turn bisexual?

General Introduction to Boundaries from Panahi Counseling:

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

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

Mariners fall flat again: 'We'll keep fighting' - The Seattle Times

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

Obsessing about clients outside of work hours.

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

Do you think your landlord should have a key to your room?

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.

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.

Solar 'cannonballs' may have stripped Mars of its water, long-awaited study reveals - Live Science

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