zerocrates parent
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.
A famous case where this came into play was one of the Infowars defamation suits. Alex Jones’s lawyer accidentally sent the families’ lawyer the full contents of a phone backup. They notified Jones’s lawyer, and gave him some time to reply. After that time elapsed, the whole dump was considered fair game.
This is the moment when that mistake was revealed in court: https://youtu.be/pgxZSBfGXUM and this is the hearing for the emergency motion to suppress that data: https://youtu.be/dKbAmNwbiMk