Some of these, I've done myself. (And I really wish I hadn't. Except the flamewar called down upon the abusive ex. That was good times.) Some of these, people have done to me. Some of these, I've seen done. Some of these, I'm very glad that I was not on the cleanup crew for. None of them are a good idea.
You know how someone on your friendslist pisses you off, and then you are moved to make a passive-aggressive rant about the thing they've just done, and then the very person who inspired your rant comes and says "Oh, I hate it when people do that!" and you have to come up with some sort of response to them? Heh. Heh. Heh.
Oh, and when someone inspires a passive-aggressive rant, and then they recognize themselves in the rant, but aren't straightforward enough to say "Look, I know this is me, cut the bullshit," so they make a passive-aggressive comment back about hoping they haven't been the inspiration for this, and then you reassure them that they're not, even though both of you know you're lying through your teeth? Yeah. Do like I say, and don't do that. I try not to do it much anymore.
Getting in public shouting matches with former members of your friendslist? Avoid. Really.
If you're going to lie, either make it a tall tale that people don't expect to be strictly the truth, or at least don't do it on something you can get caught on. ...People have ways of finding out things.
For the love of fucking Pete, don't make that nasty anonymous comment from the same IP address you left the not-nasty not-anonymous comment from five minutes earlier, especially when you leave another not-nasty not-anonymous comment five minutes later. Does "this user has enabled IP address logging" mean anything to you?
While reading your ex's unsecured journal, then cracking into your current girlfriend's journal to grant yourself access to read the entries that your ex's journal tells you that you must be missing, leaving a comment, and then deleting your extra access is a tempting thought, it leads to face-to-face defriending too.
Know who you gave your phone number out to. Don't take calls while half-asleep. Do make sure your caller ID is working. Do hang up on that creepy caller before they creep you out more.
Don't make comments that could be interpreted as mildly sarcastic or that you think the person is a moron, to someone you don't know, when coupled with that default userpic of yours that is about the trials and tribulations of dealing with morons, when speaking in your capacity as some sort of official person related to the area that you're commenting in. If you do, this is a really good time for a new default userpic. Like, right now.
Making a humorous comment that's really in very questionable, if not poor, taste, about an hour before leaving for vacation, for three weeks, in another country, where you won't have internet access, putting your friends in the very un-enviable position of attempting to clean up after you? Avoid. Really. If only because your friends are going to literally kill you when you get back. Bonus points if this somehow manages to get your friends spammed with some of the social fallout. (This is the extreme version of that thing that blows up while you're afk at work/overnight/for the weekend. No death actually occurred.)
When your friend has finally ditched her abusive ex, wait until after she has moved out before jumping all over the ass's comments in her journal. Restrain yourselves until then, and don't post about him where he can see it when he's standing over her shoulder trying to see what she's doing online. Make a note of the particularly good ideas you think of, though, because your time will come. Once he has been arrested, she has been extracted, and he has been released to his suddenly empty apartment, this is the time to descend. Do be polite, and quote his comments, correcting his spelling and grammar with careful red font before responding to them, because he does pride himself on his "highly skilled" "command" of the "language". Don't descend into the same sort of language that he has resorted to. Do wear your cleanest sockpuppets, and preemptively ban him from your own journal. Don't take it into his journal, because that's just crass. He thinks he has come armed to the battle of wits, so ten against one is perfectly fair when he's been physically shoving your friend around. You're only attacking him verbally. Do leave some for the rest; don't end the party too early. (This is one scenario where altering text color is vaguely passable, because it's already a flamewar.)
Do read the community rules before posting there.
Don't post while both angry and drunk.
Don't think that deleting a post or comment where you came off really badly is going to necessarily save you face. Someone may still have it open, or may have made a copy or screen capture. Everybody's going to stick their foot in their mouth at some point or another, and best of luck at avoiding it in the first place.
As satisfying as it may be to tell someone that you're defriending them because they're really an idiot, it'll probably start a fight, and you may not want a fight.
Why, no, Virginia, there is no such thing as "freedom of speech" in a LiveJournal community. Maintainers may run things however he, she, it, or they jolly well please, and it is entirely at their discretion whether they act with benevolence or as a petty, tin-plated dictator with the kind of behavior that would make the bastard lovechild of Caligula, Chairman Mao, Fidel Castro, and Cruella DeVille (yes, all at once) seem like a model of restraint and good leadership.
If you have a stats-tracker in a single journal entry and it gets reloaded fifteen times in an hour from a single source, don't automatically assume the worst. They're probably obsessively refreshing their friends page, not stalking you.
That sarcastic and cutting review of that journal entry that made your eyes bleed really shouldn't have been left in a place where the person who made it could see it, especially when you were just intending to vent steam that needed to be vented but you hadn't intended to aim at them. THERE IS A LOCK FEATURE.
Free tip: if you have to say "This isn't spam", it probably is. Also, saying "This isn't spam" doesn't magically make it not spam.
When the maintainers of the community put that interest in the community's interests list, that was not an invitation for you to come and invite them to try out your vaguely-related product and/or service.
If you are using LJ as a platform to solicit takers for your product and/or service, the burden of research for where an acceptable forum for inviting others to partake is, is upon you. Those posts you make to a half-billion vaguely related communities, with the disclaimer that if it's not all right to post that there, that you didn't mean to spam, and just delete it? That is an invitation to the maintainer to not only delete your post, but delete it and mark it as spam. And if you've done that to as many communities as I think you have, you're probably facing suspension, or at least a warning that if you do it again you're out.
If you're trying to communicate with one person, a shout-out in your journal might get through, but an IM or email might be more reliable. Or even a phone call. Don't depend on them reading that journal entry of yours.
On community etiquette: When you post to a community, and it doesn't quite fit the community rules, so you explain why it doesn't fit the community rules but that's OK for reason of something else that's against the community rules, and then you cap it off with something else that's not quite in line with community standards but has been okayed by something that's absolutely against community standards, and all of the rules and standards that you are violating are listed on the community profile: what the fuck. No, seriously, what the fuck. CAN YOU READ. (Some idiot was posting in metaquotes.)
Don't be too fussed if you make a post you consider important and no one comments to it.