The attorney-client privilege is a doctrine which says that you don’t have to disclose information to the other side in a lawsuit which involves discussions with your attorney. The idea is that you should be able to speak frankly to your attorney about case strategy without worrying that the other side will be able to get that information. It’s obviously pretty important to keep these discussions confidential.
However, in certain scenarios, courts have found that parties have waived their privilege. By insufficiently reviewing the electronic information they sent to the other side, the court in Victor Stanley, Inc. v. Creative Pipe, Inc. found that a company waived their privilege. They sent stuff they shouldn’t have, and now the other side could use it against them. It is another reason to have good Records Retention plans and Legal Hold plans in your company, with knowledgeable people overseeing them. Good plans help you know what you have and where you have it, which means you can search and review the information before sending it out to your opponents. You won’t need to say “Whoops! We didn’t mean to send you that stuff!”