The following code typed into Wordpress:

<div style="text-align: center; margin: auto">
<object type="application/x-shockwave-flash" style="width:425px; height:353px;" data="http://www.youtube.com/v/u-foWh9dskg">
<param name="movie" value="http://www.youtube.com/v/u-foWh9dskg" />
</object></div>

is rendered as:

<div style="text-align: center; margin: auto">
<object type="application/x-shockwave-flash" style="width:425px; height:353px;" data="http://www.youtube.com/v/u-foWh9dskg"></p>
<param name="movie" value="http://www.youtube.com/v/u-foWh9dskg" />
</object></div>

Notice the </p> just before the start of the <param> tag. This renders the XHTML invalid.

(I have attached the content of this bug report as a text file just in case the spacing is not preserved by Trac and it turns out to be important).