Security researcher Tavis Ormandy, of Google's Project Zero, notified LastPass of the bug. The flaw could have allowed hackers on malicious sites to access users' credentials entered on the previous site. Fortunately, there's no reason to believe the bug was exploited, and while it only impacted Chrome and Opera browsers, LastPass patched all browser extensions.
This isn't the first time LastPass has fixed a security flaw. A couple years ago, it found a vulnerability in its fingerprint verification. Before that, LastPass fixed security problems on Chrome and Firefox.
This doesn't mean that password managers are unreliable, just that they're not foolproof. Password managers are still a good idea, but they aren't a perfect security measure on their own. They should be used in conjunction with multi-factor authentication, and you need to keep them updated.