I've noticed it happens sporadically. I can't figure out what the reasoning might be, other than it only seems to occur in posts that have coding in them.
I hit the preview button but instead of generating a preview, an error message pops up:
"The following error or errors occurred while posting this message:
The message body was left empty."
Except the body was not empty, it was filled with code and text. No preview happens, at all. The only way around it is to hit post and hope for the best. >_>