Open the plugin and find this text on line 568:

$text = wpautop($text, 0);

Before it, add this dirty little fix:

$text = str_replace( array('<p><p>', '</p></p>'), array('<p>', '</p>'), $text);

It only works around the problem rather than fixing it, but it seems to do the job as most pages now validate. :)

EDIT: Seems WordPress originally mucked up the code that I had posted. It’s now been fixed.

The trick is to use the <!--more--> tag. However, I bet you’re thinking “but I don’t want a ‘Read More’ link!”. Well, once again, WordPress to the rescue.

For a few major versions now, you can do custom more text. You simply format it like this: <!--more Check out the rest of my post!-->. Or, to remove the link altogether while still leaving the feature intact, just use a space: <!--more -->.

Here’s an example:

This is a cool post. It has lots of interesting stuff.

More interesting stuff.

<!--more -->

This text will only show up on the single post view.

You can check it out in action as the following text will only show up in single post view:

Hey there. This is the “special” text. :)