Solution - 1

As somebody above alluded to, only administrators (and in some cases editors) can post unfiltered HTML. My guess is that your user table might be corrupted or somewhere along the way your user lost that capability. Try creating a new admin user and doing with that user. This post by Nacin explains it in more details http://wpdevel.wordpress.com/2010/12/31/the-published-exploit-for-wordpress-3-0-4-isnt-accurate/

Solution - 2

Does this happen with the TwentyTen theme?

Solution - 3

can you post the code you want to paste in some pastebin or sth?

Solution - 4

Is your user an Administrator user? Do you have any custom functions in your functions.php? I tried

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Solution - 5

I suggest using http://wordpress.org/extend/plugins/foliopress-wysiwyg/ which is a much better editor and does not strip out code the same way the default one does. then again, if you've disabled the visual editor completely and still have the problem then Im not too sure. Have you tried installing a brand new copy of WP in a temp location with a new database to rule out any database corruption?

Solution - 6

1) add this to custom field (calld it the_content_custom_fiild) 2) at the sigle.php (for blog posts) or page.php after the the_content() function runs type next 3)

Solution - 7

you should also add this to your functions.php so text is saved correctly:

// DISABLE WPTEXTURIZE 
remove_filter('the_content', 'wptexturize');
remove_filter('the_title', 'wptexturize');
remove_filter('single_post_title', 'wptexturize');
remove_filter('category_description', 'wptexturize');
remove_filter('list_cats', 'wptexturize');
remove_filter('comment_author', 'wptexturize');
remove_filter('comment_text', 'wptexturize');
remove_filter('the_rss_content', 'wptexturize');
remove_filter('comment_text', 'wptexturize');
remove_filter('the_excerpt', 'wptexturize');
// remove_filter( 'the_content', 'wpautop' );
// remove_filter( 'the_excerpt', 'wpautop' );