A opening or closing tag is found. But this tag is currently not expected.
Opening tag: the tag is not at his place.
Closing tag: the tag is not opened. It can be due to a previous error.
For a closing tag, remove the closing tag or add the missing opening tag before
BAD <b>abc</b></b> GOOD <b>abc</b>
HTML specification: http://www.w3.org/TR/html4/