Eugene Volokh writes:
In most states, you are generally free to secretly record conversations to which you are a party. Federal law likewise doesn’t ban such recordings. But in 11 states, including Washington, such recordings are illegal — and often inadmissible in court, even when they are extremely relevant. This is an exclusionary rule, but one that’s not limited to unconstitutionally obtained evidence.
Read more on WaPo Volokh Conspiracy, where Eugene discusses a case where a child molester’s conviction was overturned because his confession was recorded surreptitiously by a family member and introduced in court.