Rule of the Road No. 5: A trial lawyer must disclose and resolve conflicts of interest, or withdraw.
Trial lawyers must – absolutely must – have systems in place to check for possible conflicts of interest. Of course, there are the obvious conflicts of interest that most lawyers have sense enough to avoid (but, surprisingly, still ensnare many lawyers): you should never represent both the husband and the wife in a divorce, no matter how “amicable.” You should never represent a buyer and a seller of real property or a business, even though all they (think) they want is a “scrivener” to draft a contract. There is no such thing as a “mere scrivener.”
The smaller things are just as important: don’t take a case against your secretary’s cousin – the headaches that such a social conflict will cause are generally not worth the fee that you will earn. Exercise caution when suing former clients, even if the new matter is not substantially related to the former representation. You may be legally correct that there is no actual or technical conflict of interest that will disqualify you pursuant to Model Rule 1.9, but your former client won’t like it and will feel betrayed. In cases where there is a plausible argument that the matters are substantially related, some enterprising professional malpractice lawyer will try to find a way to show that you’re responsible for the opposing party’s bad result and put your conduct on trial, even if you’re not a party to the case.