Everybody gets a big pink warning box every so often when they try to post. It comes from the database that the blog uses to store amd keep track of all the content, including your new post. It looks grisly.
For starters, try just ignoring it. If it showed up when you clicked the Preview button, but your post looks OK to you as it's displayed in the preview page, just go ahead and click Submit. If you click the Submit button and the pink warning box shows up, do something else for a couple of minutes and then go see if your post actually ended up where it was supposed to. In my experience, things seem to actually work, even when some version of this warning appears.