> Edit: after reading up on this it looks like attorneys have strict ethical standards to not use the information (for what little that may be worth), but the Associated Press was a third party who unredacted public court documents in a separate Facebook case.

Curious. I am not a litigator but this is surprising if you found support for it. My gut was that the general obligation to be a zealous advocate for your client would require a litigant to use inadvertently disclosed information unless it was somehow barred by the court. Confidentiality obligations would remain owed to the client, and there might be some tension there but it would be resolvable.

My recollection is that it varies quite a bit between jurisdictions. The ABA's model rules require you to notify the other party when they accidentally send you something but leave unspecified what else, if anything, you might have to do.