id	summary	reporter	owner	description	type	status	priority	milestone	component	version	severity	resolution	keywords	cc
14674	HR destroys HTML	thomask		"if you add <hr /> to the post (via HTML editor, or via enhanced TinyMCE editor) followed by normal text id do not add the <p> for that paragraph, but add the finishing </p>

so it than looks like

<p>some text</p>
<hr>some other text</p>

it can be solved by adding extra line break after <hr> but after reediting the post, the linebreak disapears and must be added again. "	defect (bug)	new	normal	Future Release	Formatting	3.0.1	normal		wpautop has-patch