It can cause a rift between you if they "trusted" you and you did something that did not meet their expectations, even if it was the best thing for them or totally customary.
Many people don't get how real estate agents get paid and figure you should not collect any commission at all, and are upset if you "charge" them for the office/broker fees.
Many people also don't understand the additional costs involved in purchasing or selling, and will blame you being "gouged" with completely customary charges.
They'll also hold you responsible for lender issues, such as delays or the hassle of supplying documentation for the loan.
You'll have to take your kudos from the mirror and sleep well even though your long-time friend may feel betrayed by you. Most times, however, all is good - especially if you disclose as much to them as you do to normal clients.
I like knowing that my friends and family believe I will always have their best interests at heart and that I will do my best (which is what I do for clients, regardless of who they are anyway).
But if something should go awry, feelings may be hurt when they place their blame on me even if I don't or didn't cause the problem. One example was when there was a sewer back up in the house, before my cousin's son and his family even moved in. His wife thought they bought a lemon and that someone should have known. Even though this is an unexpected problem, and I even contributed some money towards the repairs, she continued to harbor ill feelings towards me as if it was my fault.
Problems can also arise when the agent is not selected by their friend or family member. I have had people from time to time tell me that they planned on using an agent whom they met at church. Once again, the key is the select the agent based on merit versus relationship.