If you're curious, there's an entry in
lj_support about it here.This is the text I'm proposing...feel free to suggest improvements.
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LiveJournal checks for valid HTML and LiveJournal specific tags in text that you write. If you use incorrect HTML or LiveJournal specific tags in your journal entries, comments, style, or user info bio, your text will be posted with all HTML escaped. Additionally, the following error message will appear above your text:
[Error: Irreparable invalid markup in entry. Owner must fix manually. Raw contents below.]
In order to get rid of this error and make your entry, comment, style, or bio display correctly, you will need to fix the incorrect HTML or LiveJournal specific tag(s). http://www.livejournal.com/support/faqbrowse.bml?faqid=72 contains some information about HTML. If you still cannot figure out what is wrong with your HTML, you should use your favorite search engine to find an HTML tutorial. LiveJournal Support Volunteers cannot offer HTML help. http://www.livejournal.com/support/faqbrowse.bml?faqid=75 explains how to use LiveJournal specific tags properly.
These FAQs explain how to edit your journal entries and styles:
A journal entry: http://www.livejournal.com/support/faqbrowse.bml?faqid=2
Overrides (S1): http://www.livejournal.com/support/faqbrowse.bml?faqid=124
A custom S1 style: http://www.livejournal.com/support/faqbrowse.bml?faqid=128
A custom S2 style: http://www.livejournal.com/support/faqbrowse.bml?faqid=177
Comments cannot be edited, so you will have to delete and repost the comment using valid HTML and LiveJournal specific tags. http://www.livejournal.com/support/faqbrowse.bml?faqid=15 explains how to delete comments.
To edit your user info bio, go to http://www.livejournal.com/editinfo.bml.