Sometimes when using the new code tags, only the first line of the code gets wrapped in the code block, and the rest shows up outside of it.
Trying to reproduce the bug, I think this is how it occurs:
Have your editor in WYSIWYG mode, and manually type [ code ] and [ /code] around a multiline group of text:
Hit "Preview", and see that the preview looks fine, but the new code in the edit box is messed up:
Note: You don't have to view the preview for the bug to occur, hitting "post" instead of "preview" will also cause the bug (or it may occur only after trying to edit a post, I'm not sure).