diff --git modules/filter/filter.module modules/filter/filter.module
index 24c64bd..d66826b 100644
--- modules/filter/filter.module
+++ modules/filter/filter.module
@@ -1590,11 +1590,12 @@ function _filter_autop($text) {
   // All block level tags
   $block = '(?:table|thead|tfoot|caption|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|select|form|blockquote|address|p|h[1-6]|hr)';
 
-  // Split at opening and closing PRE, SCRIPT, STYLE, OBJECT tags and comments.
-  // We don't apply any processing to the contents of these tags to avoid messing
-  // up code. We look for matched pairs and allow basic nesting. For example:
+  // Split at opening and closing PRE, SCRIPT, STYLE, OBJECT, IFRAME tags
+  // and comments. We don't apply any processing to the contents of these tags
+  // to avoid messing up code. We look for matched pairs and allow basic
+  // nesting. For example:
   // "processed <pre> ignored <script> ignored </script> ignored </pre> processed"
-  $chunks = preg_split('@(<!--.*?-->|</?(?:pre|script|style|object|!--)[^>]*>)@i', $text, -1, PREG_SPLIT_DELIM_CAPTURE);
+  $chunks = preg_split('@(<!--.*?-->|</?(?:pre|script|style|object|iframe|!--)[^>]*>)@i', $text, -1, PREG_SPLIT_DELIM_CAPTURE);
   // Note: PHP ensures the array consists of alternating delimiters and literals
   // and begins and ends with a literal (inserting NULL as required).
   $ignore = FALSE;
@@ -1602,9 +1603,14 @@ function _filter_autop($text) {
   $output = '';
   foreach ($chunks as $i => $chunk) {
     if ($i % 2) {
-      // Opening or closing tag?
-      $open = ($chunk[1] != '/' || $chunk[1] != '!');
       $comment = (substr($chunk, 0, 4) == '<!--');
+      if ($comment) {
+        // Nothing to do, this is a comment.
+        $output .= $chunk;
+        continue;
+      }
+      // Opening or closing tag?
+      $open = ($chunk[1] != '/');
       list($tag) = preg_split('/[ >]/', substr($chunk, 2 - $open), 2);
       if (!$ignore) {
         if ($open) {
@@ -1613,7 +1619,7 @@ function _filter_autop($text) {
         }
       }
       // Only allow a matching tag to close it.
-      elseif ((!$open && $ignoretag == $tag) || $comment) {
+      elseif (!$open && $ignoretag == $tag) {
         $ignore = FALSE;
         $ignoretag = '';
       }
diff --git modules/filter/filter.test modules/filter/filter.test
index e8b8058..cb12ec9 100644
--- modules/filter/filter.test
+++ modules/filter/filter.test
@@ -833,6 +833,25 @@ class FilterUnitTestCase extends DrupalUnitTestCase {
       '<blockquote><pre>aaa</pre></blockquote>' => array(
         "<blockquote><pre>aaa</pre></blockquote>" => TRUE,
       ),
+      "<pre>aaa\nbbb\nccc</pre>\nddd\neee" => array(
+        "<pre>aaa\nbbb\nccc</pre>" => TRUE,
+        "<p>ddd<br />\neee</p>" => TRUE,
+      ),
+      // Comments remain unchanged and subsequent lines/paragraphs are
+      // transformed normally.
+      "aaa<!--comment-->\n\nbbb\n\nccc\n\nddd<!--comment\nwith linebreak-->\n\neee\n\nfff" => array(
+        "<p>aaa</p>\n<!--comment--><p>\nbbb</p>\n<p>ccc</p>\n<p>ddd</p>" => TRUE,
+        "<!--comment\nwith linebreak--><p>\neee</p>\n<p>fff</p>" => TRUE,
+      ),
+      // Check that a comment in a PRE will result that the text after
+      // the comment, but still in PRE, is not transformed.
+      "<pre>aaa\nbbb<!-- comment -->\n\nccc</pre>\nddd" => array(
+        "<pre>aaa\nbbb<!-- comment -->\n\nccc</pre>" => TRUE,
+      ),
+      // Bug 810824, paragraphs were appearing around iframe tags.
+      "<iframe>aaa</iframe>\n\n" => array(
+        "<p><iframe>aaa</iframe></p>" => FALSE,
+      ),
     );
     $this->assertFilteredString($filter, $tests);
 
@@ -1464,7 +1483,7 @@ www.example.com with a newline in comments -->
         if (!$success) {
           $this->verbose('Source:<pre>' . check_plain(var_export($source, TRUE)) . '</pre>'
             . '<hr />' . 'Result:<pre>' . check_plain(var_export($result, TRUE)) . '</pre>'
-            . '<hr />' . ($is_expected ? 'Found:' : 'Not found:')
+            . '<hr />' . ($is_expected ? 'Expected:' : 'Not expected:')
             . '<pre>' . check_plain(var_export($value, TRUE)) . '</pre>'
           );
         }