Namely, people getting randomly redirected to other people's accounts. Logged in as that person, able to view messages and change settings. This is a serious security issue for anyone using LJ to buy/sell items who may have paypal addresses, e-mail addresses, or mailing addresses in their inbox. It's been mentioned several times in the new release post.

If you have any information in your inbox that you wouldn't want another person to see, I'd suggest saving it elsewhere and deleting it from your inbox, especially if you choose to stay logged in.

Also, apparently LJ has changed the way they handle cookies today. It was supposed to be a minor thing, but as usual they've tried to fix one thing and broken several others. It's affected some plugins (like LJlogin). I'm not sure if the two issues may be related, but just throwing this out there FOR YOUR GENERAL KNOWLEDGE.