I agree that's a possibility i.e. false positive emails.

Counterpoints:

- LastPass officially responded to this story by saying that all of our passwords were compromised elsewhere and then someone attempted to login with those i.e credentials stuffing.

If it was a false positive email, it would have been easier to say that and a lesser reputation hit on them. Right now, they're saying "oh yeah those passwords were valid -- it's just that the attacker got them somewhere else, we weren't breached."

- The (now 3!) twitter reports mentioned in https://news.ycombinator.com/item?id=29719033 point out that someone attempted to login again after they changed their password. They received the "Someone just used your master password" email a 2nd time.

If those emails are not false positives (per LastPass), how could that have happened?

That's what I was hoping, but I tested it.

Used a VPN endpoint in a country that would surely get blocked. Attempted to login with wrong password, did not receive email. Logged in with correct passphrase, received email. Both scenarios looked the same on the login screen, so there wasn't any indication that I logged in with the correct password if I were the attacker

edit: last night they released something that says they sent some of those emails in error. I hope that is the case, but it's still not very re-assuring. I went through and changed all of my financial/critical accounts yesterday in precaution, today my bank account was locked out from brute force. Could be a coincidence, but that is the first time it's ever happened with that account

Seems like this was infact the case: "Our investigation has since found that some of these security alerts, which were sent to a limited subset of LastPass users, were likely triggered in error. As a result, we have adjusted our security alert systems and this issue has since been resolved."

Agree here.