It means you screwed something up when you wrote the .htaccess file.
If theres nothing private in the file (passwords or something you don't want revealed), post the contents and someone might be able to see the mistake.
if you're using a rewrite though (modrewrite, rewriterule, rewritecond if I remember correctly), that'll probably be whats causing the issue. I can't remember ever doing a rewrite where it worked the first time XD