Clients who don't get everything they wanted. Clients who think they shouldn't have to pay everything you asked them to. Clients whose friends -- armchair lawyers, all -- tell them to.
What do all of those people have in common? They're in the class of "people who are most likely to file a bar complaint against you." The bar complaint is a Sword of Damocles that clients think they can hang above attorneys to get what they want, including out of a fee agreement.
So what should you do if you're on the receiving end of such a complaint?
Just Do What You're Told
In most states, after a client files a bar complaint, the state bar sends the attorney a letter asking for information. At this stage, you don't have to hire counsel or really do anything except respond to the letter.
In the case of a fee dispute, for example, a letter from the state bar might ask for copies of the fee agreement (which you put in writing, right?) and any evidence -- in the form of invoices, timesheets, and canceled checks -- showing how much time you spent on the case and how much the client paid. The importance of record-keeping should go without saying, but for small firms (and especially solos), record-keeping is sometimes a distant second in importance to keeping up with client business.
What shouldn't you do? Contact the client, for one. That could be construed as harassment (even if it's not) just because it happens chronologically after the filing of the complaint. It's best to respond only to the state bar, and then, only when asked. You also shouldn't ignore a request from the state bar; that in itself is a separate ethical violation, whether or not the bar complaint was meritorious.
Provide everything the bar asks for, and do it on time. Oh, and don't try to get cute with the rules by omitting things that, though not specifically asked for, are necessary for complete honesty. Bar lawyers will see right through that and count it as a strike against you.
If you anticipate that a client might be trouble, there are some things you can do pre-emptively. Document everything you say and do with the client in a Word document somewhere so that if the bar complaint happens, you have a contemporary narrative of events rather than a half-remembered one. Whenever you have a phone conversation with the client, send an email summarizing the things you talked about so the client can't later deny that you told him something, when in fact you did.
And, not to blame the victim, but don't give clients a reason to be unhappy. Sure, some of them are naturally going to be unhappy, but things like providing a written fee agreement, explaining how the lawyer/client relationship works, and setting expectations early can placate the rest of the clients who otherwise wouldn't file a complaint.