One of the things they don't mention in law school is that being an attorney sometimes requires a delicate manner -- especially when it comes to telling clients that they're wrong.
Information about "the law" is everywhere, and some of it is incorrect. If it hasn't happened to you yet, it's only a matter of time before a client comes in who thinks he "knows" exactly what the law is.
Except he doesn't. What he's heard on TV or the Internet is wrong, or out of date, or not applicable to his situation. Now it's your job to break it to him. Not all situations are the same, so here are a few suggestions about how to keep things civil:
Explain that the law was recently updated. If the law your client is quoting is out of date, gently explain that it was recently revised, and then explain its current status. That acknowledges the client's understanding, but still gets the real information out.
Suggest alternatives. Rather than outright saying the client is incorrect, give other ways to tackle their legal issues. If you make them sound attractive and give examples or reasons why the client's suggestion might fail, you may not have to come out and say that the client's original idea was wrong.
Ask for the client's reasoning. Another way to sidestep actually having to say "You're wrong," knowing your client's motives gives you a way to introduce other ideas. Once you know what they were hoping to accomplish, present legally feasible alternatives that fulfill those needs.
Cite high-profile cases. Everyone wants to be like someone successful. If you have success stories from your own cases, or even lessons from other famous cases, use them to convince a client to do things your way. There's a good chance the only reason the client brought up the "wrong way" is because it sounded like a tried-and-true idea.
Apply it to the case. Showing often works better than telling to bring someone around to your way of thinking. If your client wants to pursue a poor strategy, ask what they would do when it's applied to unhelpful facts in the case. Hopefully they'll come to the right conclusion on their own, once you show them the error of their ways.
Blame the judge or jury. To maintain a good relationship with your client, you might want to avoid saying "I think you're wrong." Instead, phrase your concern in terms of what judges and juries usually find or are generally convinced of. Even if your client still holds his mistaken belief, he may change his mind in order to better convince the jury.
Assess the situation, then be direct. Sometimes the direct approach of "You're wrong" works best, especially for clients who are open to your expertise. Just be careful about using this method, since some clients will never react well to being told they've made a mistake.