Here's a handy tip for those new to LJ or just forgetful. When you use LJ's (clunky) filters system to make posts that only some people can see, make this clear in the post, possibly in the first line.
Yes indeed you can use LJ features to set a post viewable by everyone (public), just you (private), just your livejournal "friends" list, or an arbitrary list of users (filtered). Many people use this last option to segregate their posts by audience or content, protect personal or secret information, or plan surprise events for their loved ones and direst enemies.
It is particularly in these last few cases that it is quite helpful to declare the filter and explain its' use. Any of various methods are fine to use so long as your intent is communicated. It's pretty common for people in long term relationships to have an everyone-but-alice filter (where alice is their significant other) for grousing, surprise parties, pre-nuptial planning, and such like. In these cases it is particularly important that those reading the post know that the target is not reading it, and whether or not it is to be secret from them. It is common to present a justification along with the declaration of the filtered group, but this is not strictly required.
Examples abound from the common "planning a birthday / anniversary / surprise funeral" to the more melodramatic. In extreme cases it might seem easier to just open up an additional LJ or community for the purposes of the exclusionary discussion. This is probably overkill. On the other hand I did exactly that with the pornographic material *shrug*, but I am notably prudish.
Thanks for reading and thinking. This, by the way, is a public post.