=== modified file 'includes/form.inc'
--- includes/form.inc	2009-12-17 21:59:31 +0000
+++ includes/form.inc	2009-12-29 10:23:42 +0000
@@ -879,15 +879,16 @@ function drupal_redirect_form($form_stat
  *   theming, and hook_form_alter functions.
  */
 function _form_validate(&$elements, &$form_state, $form_id = NULL) {
-  // Also used in the installer, pre-database setup.
-  $t = get_t();
-
   // Recurse through all children.
   foreach (element_children($elements) as $key) {
-    if (isset($elements[$key]) && $elements[$key]) {
-      _form_validate($elements[$key], $form_state);
-    }
+    _form_validate($elements[$key], $form_state);
+  }
+  if (_form_validate_skip($elements, $form_state)) {
+    return;
   }
+  // Also used in the installer, pre-database setup.
+  $t = get_t();
+
   // Validate the current input.
   if (!isset($elements['#validated']) || !$elements['#validated']) {
     if (isset($elements['#needs_validation'])) {
@@ -947,6 +948,46 @@ function _form_validate(&$elements, &$fo
 }
 
 /**
+ * Check if the validation of an element needs to be skipped.
+ *
+ * Multistep forms not wanting to validate the whole form can set the
+ * #validate_parents property on buttons to avoid the validation of some
+ * elements. For example, pressing the Previous button should not fire
+ * validation errors just because the current step has invalid values.
+ *
+ * If this property is set on the clicked button the form level #validate and
+ * #submit handlers won't fire. Button level #validate and #submit handlers
+ * will fire and there should be extreme care taken because
+ * $form_state['values'] will contain values not validated. However, this
+ * typically is not a problem as these buttons (like Previous or Add more) do
+ * not persist anything. Do not use this property if the button triggers saving
+ * to the database.
+ *
+ * This property is a list of form_state['values'] keys. For example
+ * $form['submit]['#validate_parents'] = array(
+ *   array('foo', 'bar'),
+ *   array('step1'),
+ * );
+ *
+ * will validate $form_values['step1']['choice'] as the first key is step1.
+ * Anything under $form_values['step2'] won't be validated. $form['foo'] won't
+ * be validated but everything under $form['foo']['bar'] will be.
+ */
+function _form_validate_skip($elements, $form_state) {
+  // The complete form does not have #parents set. However, the complete form
+  // validation fires #validate handlers set on the clicked button so in this
+  // case FALSE must be returned.
+  if (isset($form_state['clicked_button']['#validate_values']) && isset($elements['#parents'])) {
+    foreach ($form_state['clicked_button']['#validate_values'] as $section) {
+      if (array_slice($elements['#parents'], 0, count($section)) !== $section) {
+        return TRUE;
+      }
+    }
+  }
+  return FALSE;
+}
+
+/**
  * A helper function used to execute custom validation and submission
  * handlers for a given form. Button-specific handlers are checked
  * first. If none exist, the function falls back to form-level handlers.
@@ -967,8 +1008,10 @@ function form_execute_handlers($type, &$
   if (isset($form_state[$type . '_handlers'])) {
     $handlers = $form_state[$type . '_handlers'];
   }
-  // Otherwise, check for a form-level handler.
-  elseif (isset($form['#' . $type])) {
+  // Otherwise, check for a form-level handler. If only some elements were
+  // validated because #validate_values was set then neither the validate nor
+  // the submit handler should fire.
+  elseif (isset($form['#' . $type]) && empty($form_state['clicked_button']['#validate_values'])) {
     $handlers = $form['#' . $type];
   }
   else {
@@ -1006,7 +1049,7 @@ function form_execute_handlers($type, &$
  * @param $message
  *   The error message to present to the user.
  * @return
- *   Return value is for internal use only. To get a list of errors, use 
+ *   Return value is for internal use only. To get a list of errors, use
  *   form_get_errors() or form_get_error().
  */
 function form_set_error($name = NULL, $message = '') {
@@ -3261,7 +3304,7 @@ function batch_process($redirect = NULL,
   $batch =& batch_get();
 
   drupal_theme_initialize();
-  
+
   if (isset($batch)) {
     // Add process information
     $process_info = array(
@@ -3276,7 +3319,7 @@ function batch_process($redirect = NULL,
     );
     $batch += $process_info;
 
-    // The batch is now completely built. Allow other modules to make changes to the 
+    // The batch is now completely built. Allow other modules to make changes to the
     // batch so that it is easier to reuse batch processes in other enviroments.
     drupal_alter('batch', $batch);
 

=== modified file 'modules/field/field.form.inc'
--- modules/field/field.form.inc	2009-12-21 13:47:31 +0000
+++ modules/field/field.form.inc	2009-12-29 09:33:41 +0000
@@ -214,6 +214,7 @@ function field_multiple_value_form($fiel
           '#name' => $field_name . '_add_more',
           '#value' => t('Add another item'),
           '#attributes' => array('class' => array('field-add-more-submit')),
+          '#validate_values' => array(array($field_name)),
           // Submit callback for disabled JavaScript.
           '#submit' => array('field_add_more_submit'),
           '#ajax' => array(

=== modified file 'modules/poll/poll.module'
--- modules/poll/poll.module	2009-12-14 20:38:15 +0000
+++ modules/poll/poll.module	2009-12-29 09:27:07 +0000
@@ -273,6 +273,7 @@ function poll_form($node, &$form_state) 
     '#value' => t('More choices'),
     '#description' => t("If the amount of boxes above isn't enough, click here to add more choices."),
     '#weight' => 1,
+    '#validate_values' => array(array('choice')),
     '#submit' => array('poll_more_choices_submit'), // If no javascript action.
     '#ajax' => array(
       'callback' => 'poll_choice_js',
@@ -901,4 +902,3 @@ function poll_user_cancel($edit, $accoun
       break;
   }
 }
-

=== modified file 'modules/simpletest/tests/form.test'
--- modules/simpletest/tests/form.test	2009-12-17 17:18:02 +0000
+++ modules/simpletest/tests/form.test	2009-12-29 10:24:53 +0000
@@ -231,6 +231,20 @@ class FormValidationTestCase extends Dru
     $this->assertNoFieldByName('name', t('Form element was hidden.'));
     $this->assertText('Name value: element_validate_access', t('Value for inaccessible form element exists.'));
   }
+
+  /**
+   * Test button press validates only certain values.
+   */
+  function testValidateValues() {
+    $edit = array('test' => 'pass');
+    $path = 'form-test/validate-values';
+    $this->drupalPost($path, $edit, t('Partial validate'));
+    $this->assertNoText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertText('Test is validated');
+    $this->drupalPost($path, $edit, t('Full validate'));
+    $this->assertText(t('!name field is required.', array('!name' => 'Title')));
+    $this->assertText('Test is validated');
+  }
 }
 
 /**

=== modified file 'modules/simpletest/tests/form_test.module'
--- modules/simpletest/tests/form_test.module	2009-12-17 17:18:02 +0000
+++ modules/simpletest/tests/form_test.module	2009-12-29 10:25:20 +0000
@@ -17,6 +17,13 @@ function form_test_menu() {
     'access arguments' => array('access content'),
     'type' => MENU_CALLBACK,
   );
+  $items['form-test/validate-values'] = array(
+    'title' => 'Form validation values test',
+    'page callback' => 'drupal_get_form',
+    'page arguments' => array('form_test_validate_values'),
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
 
   $items['form_test/tableselect/multiple-true'] = array(
     'title' => 'Tableselect checkboxes test',
@@ -203,6 +210,37 @@ function form_test_validate_form_validat
   }
 }
 
+function form_test_validate_values($form) {
+  $form['title'] = array(
+    '#type' => 'textfield',
+    '#title' => 'Title',
+    '#required' => TRUE,
+  );
+  $form['test'] = array(
+    '#type' => 'textfield',
+    '#element_validate' => array('form_test_validate_test_element'),
+  );
+  $form['buttons']['partial'] = array(
+    '#type' => 'submit',
+    '#validate_values' => array(array('test')),
+    '#value' => t('Partial validate'),
+  );
+  $form['buttons']['full'] = array(
+    '#type' => 'submit',
+    '#value' => t('Full validate'),
+  );
+  return $form;
+}
+
+/**
+ * Element handler for the test element.
+ */
+function form_test_validate_test_element($form, $form_state) {
+  if ($form_state['values']['test'] == 'pass') {
+    drupal_set_message('Test is validated');
+  }
+}
+
 /**
  * Create a header and options array. Helper function for callbacks.
  */
@@ -895,7 +933,7 @@ function form_test_state_persist($form, 
 
 /**
  * Submit handler.
- * 
+ *
  * @see form_test_state_persist()
  */
 function form_test_state_persist_submit($form, &$form_state) {
@@ -905,7 +943,7 @@ function form_test_state_persist_submit(
 
 /**
  * Implements hook_form_FORM_ID_alter().
- * 
+ *
  * @see form_test_state_persist()
  */
 function form_test_form_form_test_state_persist_alter(&$form, &$form_state) {