If you have the <lj user> tag more than once in your entries, with a user first and then a community, the RTE is eating the data between those tags.
This is if you are doing it all in the RTE, switch over to the RTE to post, switch to HTML mode to post, switch briefly into the RTE to look at it, have it in the RTE at all, either composing or editing ... in short, this is a bad, bad, bad bug if you use the <lj user> tag and the RTE.
Keep an eye on lj_releases for word on when it's safe to use the RTE again. (I will probably forget to tell you, because I loathe the thing like burning, and THIS IS JUST ANOTHER SIGN, MY FRIENDS, A SIGN OF THE TIMES. *marches about with cardboard "TITS AGAINST THE RTE!" sign, mumbling and shouting*)