[dghlsakjg]

Morally gray, sure.

Legally, I can't see what's wrong with using information that you have, even if the other party didn't intend for you to have it. Lawyers themselves will use information in court that was accidentally sent to them by a counter-party, and that the other lawyer never intended them to have.

[Spooky23]

It may be technically an issue with some government bids, if you need to file an affidavit certifying you had no such knowledge.

But how would they prove it? And, doing so would reveal that they fucked up in the first place by sending it to you.