There are limits on the number of posts that can be waiting in a community's moderation queue. If a user tries to post to the community after these limits have been reached, he/she will receive an error message stating "Maximum queued posts for this community+poster combination reached," and the entry will not be sent to the moderation queue or to the community. The maintainer will need to approve or reject moderated posts in order for the user to be able to post more entries to the community. The final paragraph of the FAQ on LiveJournal's data limitations (http://www.livejournal.com/support/faqbrowse.bml?faqid=148) describes these limits.
I know the wording is a bit off, so feel free to tear apart and stitch back together.
