It seems many people haven't really thought through the implications of this new feature, and some people might not be aware of it. So.
LJ has introduced a feature where, if you link your LJ account to your Twitter or Facebook account, you can publish all your entries and/or comments to that remote site. It will include the title, the first bunch of words, and a link to the entry or comment.
The settings don't completely suck; even if you turn on crossposting by default, it will be turned off by default for comments to locked entries. But it's not hard to turn on (just a checkbox), even by accident (a checkbox interjected in the tab order that's muscle memory for some people). I think the greater risk here is going to be people who don't have crossposting comments on by default, but do choose to crosspost comments semi-frequently; for them, crossposting their comment to a locked entry won't feel any different than crossposting to an open entry. As much as I hate them, an "are you sure" popup might have been appropriate here, just for locked posts.
So, here's the thing. If you quote when you comment, and you crosspost, then you're crossposting whatever you quoted. Even if you don't, your comment is likely to be indicative of what was being discussed and possibly some of the content, and it will include a link which indicates the presence of a locked entry on that topic.
No real privacy on the Internet, information wants to be free, blah, blah, blah. I know none of you wants to be the jerk who reveals something someone preferred to keep to a select group [locked post] to a broader audience, so tread very carefully with this new feature. If you even turn it on.