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

Off the top of my ancient head:

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.

Nutritionists Are Sharing Alcohol “Rules” You Should Really Be Following, Including Women Having No More Than One Drink Per Day - BuzzFeed

Serious disappointment when the client cancels a session.

Obsessing about clients outside of work hours.

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

Has Messi scored against the Buffon?

General Introduction to Boundaries from Panahi Counseling:

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.

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

Murder, Sexual Assault, And Massive Fraud: 15 Celebrities Who Are In Jail For A Long Time - BuzzFeed

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

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

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

Breast transplanted to replace cancerous one in 'world first' operation - BBC

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