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.

General Introduction to Boundaries from Panahi Counseling:

Why does my vagina and around my butthole itch? I don't have weird discharge and I'm still a virgin.

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.

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

Lower 'bad' cholesterol and higher fat-transport markers linked to less Alzheimer's risk - Medical Xpress

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

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.”

Which is better, a naked picture of some one you know or porn videos?

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

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

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

New features added to Nintendo Today! - News - Nintendo Official Site - Nintendo

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.