It depends on if they were caught, the level of harm caused, and the attorney they hire to sue the pants off the person who breached. 

The attorney with the bigger bat usually wins. 

It is deemed  a breach of fiduciary duty, and the harmed party can sue for damages.

An excellent attempt to change up the perpetual "which fiduciary duty..." question.

One may face legal consequences since these duties are mandated by law.

Nothing good.  

I say, trouble right here in River City"

Why?  What did you do?

You will be put on a broom and asked to fly off into the sunset!

The way this question is framed, I will go with the explanation of Kathleen Daniels, Probate & Trust Specialist. 

When fiduciary duties are not fulfilled will cause liability problems

The real answer to this question lies in exploring the subject matter. The question itself is like a door-way. When we meet-up with a crook or a liar, those uncomfortable feelings that come forth are contrary to whats right or good thus we yearn for solution or closure from the episode. Honesty, integrity, character and applying the golden rule all serve as a type of food for the soul and produce a well-being, something money cannot buy. In all relationships, we yearn for decency, intimacy and prosperity....

Your troubles could include fines, loss of license and jail time. Your legal defense costs could be in the thousands.

Very unhappy client.

Client lodges a complaint with the local Realtor Board and you are judged at a hearing to have harmed the client.

Client reports your activity to the local government agency that is in charge of licensee oversight. You lose your license.

Client files a police report. You are charged with a crime, tried convicted an go to jail.

Client files a civil suit against you and you lose and owe the client a bunch of money.

Of course, not all who stray from their fiduciary duties suffer the above consequences. Do you want to try your luck?