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This is either a big privacy bug, or a "feature" that people are going to slam into very quickly. If you have a private post on G+, people can currently make that post viewable to others simply by +'ing their names in the comments. Both the original poster and any commenters can do this.
Some important things to remember about post access, which I suggest you read if you care about the privacy of your postings here, as it's not all immediately evident:

When someone is +mentioned (for example, +Sierra Kempster ), in a posting that person then have access to the post and its comments, even if they were not initially specified as having access to the post.

When someone is +mentioned by you in a comment to a post the person mentioned then has access to the post and its comments, even if they were not initially specified as having access to the post by the original poster and even if you later delete the comment that mentions that person.

When someone is +mentioned by someone else (other than the original poster) in a comment to a post the person mentioned then has access to the post and its comments, even if they were not initially specified as having access to the post by the original poster and even if the original poster later deletes the comment that mentions that person.

I'm not sure that the behavior specified in the third case above is desirable.

For example, let's say I wanted to throw someone (for example, David Schlesinger) a surprise party, and make a post to plan it with a limited circle of our mutual friends and acquaintances. If any one of those friends or acquaintances then +mentions +David Schlesinger in a comment, he now has access to the post and all its comments, with no way to remove it. He will then know all about what we planned to have popping out of that giant cake; how sad!

I can think of a few considerably more malicious scenarios.

I'm not sure how the best way to go about implementing this in the UI, but it seems like it would be useful, and perhaps vital, for an original poster to be able to remove access to a given post that was later added by someone other than the original post through a +mention in comments.

What do you think?

(Many thanks to +Wade Aaron Inganamort, +Sierra Kempster, and +Autumn Tyr-Salvia for helping me test some of these scenarios out last night. This has been sent as feedback, but I'm not sure the G+ team sees this issue as a bug.)
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Thanks for the share!

One idea that I've gotten on this (from +Rhett Aultman) would be differentiating in behavior between @mentioning and +mentioning someone, with one notifying and granting access to a discussion, and the other not doing so.

This partial solution might count as "too complicated" though....
Hmm...I intuitively knew that this was how Google+ worked, and I'm not sure why. Nevertheless, definitely sharing it.
+Jillian C. York that a commenter could force a revelation of a closed, unshareable thread simply by mentioning somebody? What does "unshareable" mean in this instance, if so? (Paging +Rod Begbie also)
+Jillian C. York: Yeah, the +mentioning-to-add-to-conversation came naturally to me as well; the bit about not being able to remove someone once they'd been mentioned didn't seem quite as intuitive, so I figured it was worth publicizing.
Do you think this is different than someone forwarding an email that you've sent to them?
I think it's a bit more like putting someone's email address in the body of your email, and having the whole thread magically sent to them.

I think there's an intuitive middle-ground to this, btw: one of the key questions with "reshare disabled" is the idea that commenters are dissuaded from reforwarding. It would make sense that they couldn't call people to the thread under those circumstances.
Ok, thanks for the insight and feedback. I'll take that back to the team!
I'd also add that maybe the intuitive split here is between using + to "summon" people, and using + to refer to people. Maybe one of the reasons I'm more surprised by this than +Fox Magrathea Circe and +Jillian C. York is that I use + so people can click and find out who it is I'm talking about. That seems to be a natural use, and also exactly the time you might not want somebody to see your thread. To be 20th century Internet about it, it's Cc: headers vs hyperlinks.
Yeah, if you have an email thread and somebody adds somebody to the cc list (and all are doing group reply) the typical behaviour is they get to read the thread so far (because it is in included text in most emails) and they get ccd on future branches of that thread, but they don't get the other branches of the thread, and anybody can pull them off the cc list (as they can do to anybody.) E-mail is complex though, and it is not inherent that the old thread stuff is included. So "let me add Fred to this thread" is something you like to do in E-mail but sometimes you don't like that people do it.
That makes sense +Danny O'Brien . Like, what if you're planning someone's surprise party? That would be a serious bummer ;)
I guess it's a matter of trusting people and learning, but it's a tricky issue! Again, thanks for raising it.
Yes, it is tricky! It feels like "disable reshare" is turning out to bear a lot more responsibility than it was originally expected to hold.
I think that power users (like us) will be able to grasp that +user invites them to the thread, and have the foresight to disable reshare. I don't, however, think that most normal users will be able to do this. I actually think that +invite and @refer need to be separate. If this is changed during beta, the power users will learn. But I don't think normal people will be able to handle this subtlety.

(also it needs to not be so friggin hard to not reference someone once you've typed @. There are users with names like invite and refer and by default if I keep typing it links to them.)
I am somewhat ashamed that everyone else appears to use the Internet for planning birthday parties, whereas I just use it for backbiting gossip
+1 for calling the + action "to summon"... I think that you should not have to summon someone in order for their name to be hyperlinked. Any proper name in text should automagically be hyper.
If all names are hyperlinked, then one is missing a signifier that that person now has access to the conversation, and + or @ in front is a much weaker signifier than the name being hyperlinking. And there are going to be times when I was to refer to Danny O'Brien without him necessarily being aware of it. (Just don't tell him that!)
The simplest solution to this is to remove the ability to make posts "private". Maybe Google+ isn't the right place to have private conversations. It's more like a forum than Facebook is.

Obviously they still haven't made it clear what Google+ is, apart from a "social something". Posting content pretty much always involves clicking a button labelled "Share", so this could turn out to be more like a replacement for services like and Tumblr, than a place to chat with friends.
"The simplest solution to this is to remove the ability to make posts 'private'."

Gee, that seems kind of like throwing the baby out with the bathwater. And then burning down the house. If you want to destroy this service quickly, force everyone to have to communicate everything with everyone.
I'm not at all sure it would destroy the service. There are plenty of places already where you can go and have private discussions, there aren't any really good places for public socialising.

Also it's in no way a case of "having to communicate everything with everyone". You'd still be able to target things, you just wouldn't be able to hide things.
Well, that would kill it for me, for sure. Not everything is public, and if I need to go to another service every time I don't want something to be potentially viewable by everyone, I'd simply stay on another service.
That's okay though. Google+ doesn't have to appeal to everyone. Facebook doesn't appeal to me because it's so private.
"Google+ doesn't have to appeal to everyone."

I'd imagine the Google+ product marketing team might have a different opinion on this.
I'd imagine that the Google+ product marketing team is extremely small (if it even exists) and has very limited influence in a company like Google.
Ed, I will bet you any amount of money that what you're proposing will never happen. And for good reason.
I'm startled by +Ed Singleton's comments. The #1 reason that I'm interested in plus is that the circle model makes more targeted sharing -- telling your local pals these things and your family those things, rather than sharing with everyone you know, or nobody -- simple and intuitive. And "so private" is a very odd descriptor to apply to Facebook.
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