Spoiler manners

Reading a discussion linked from daily_snitch, I read a major spoiler for an episode of Lost. Thoroughly pissed off, because I've been very careful, and didn't expect it in a HP discussion, I posted a "thanks for the spoiler" message. The journal owner posted back that her journal is a "spoiler haven" (which, to be fair, she has posted clearly on her info page) and it happened months ago anyway and I shouldn't read LJ if I don't want to be spoiled.

Now, I am sorry for posting to her journal, as she isn't someone I know, but my dilemma is that, coming from daily_snitch, I had no way of knowing that her journal was a "spoiler haven". All our shows are weeks, sometimes months, behind US schedules, and I am careful not to join, say, Dr. Who or Lost lists, or friend people who spoil, but I can't avoid all fandoms and all LJs forever. I've already stopped going to the Yahoo homepage because of spoilers.

Really, I'm just posting this out of frustration. I don't participate in any TV-fandoms because it's simply impossible (I can't look up any information about episodes until the season is over, for example) but to be spoilered by reading another fandom entirely? I really don't know what to do.