diff --git a/feeds_ui/feeds_ui.admin.inc b/feeds_ui/feeds_ui.admin.inc
index f975ffe..c12cb3a 100644
--- a/feeds_ui/feeds_ui.admin.inc
+++ b/feeds_ui/feeds_ui.admin.inc
@@ -401,6 +401,15 @@ function feeds_ui_edit_page(FeedsImporter $importer, $active = 'help', $plugin_k
   $info['actions'] = array(l(t('Change'), $path . '/processor'));
   $config_info[] = $info;
 
+  // Validate configuration.
+  $errors = $importer->validateConfig();
+  if (count($errors)) {
+    $message = t('There are some issues with the importer configuration: !errors', array(
+      '!errors' => theme('item_list', array('items' => $errors)),
+    ));
+    drupal_set_message($message, 'warning');
+  }
+
   return theme('feeds_ui_edit_page', array(
     'info' => $config_info,
     'active' => $active_container,
@@ -1186,12 +1195,28 @@ function feeds_ui_importer_import_validate($form, &$form_state) {
   }
 
   if (!$form_state['values']['bypass_validation']) {
+    $errors = array();
+
+    // @todo Move to FeedsImporter::validateConfig()?
     foreach (array('fetcher', 'parser', 'processor') as $type) {
       $plugin = feeds_plugin($feeds_importer->config[$type]['plugin_key'], $feeds_importer->id);
       if (get_class($plugin) == 'FeedsMissingPlugin') {
-        form_error($form['importer'], t('The plugin %plugin is unavailable.', array('%plugin' => $feeds_importer->config[$type]['plugin_key'])));
+        $errors[] = t('The plugin %plugin is unavailable.', array('%plugin' => $feeds_importer->config[$type]['plugin_key']));
+      }
+    }
+
+    // Other validation rules.
+    $importer = feeds_importer($feeds_importer->id);
+    $importer->setConfig($feeds_importer->config);
+    foreach (array('fetcher', 'parser', 'processor') as $type) {
+      $plugin = feeds_plugin($feeds_importer->config[$type]['plugin_key'], $feeds_importer->id);
+      if (get_class($plugin) != 'FeedsMissingPlugin') {
+        $importer->setPlugin($feeds_importer->config[$type]['plugin_key']);
+        $importer->$type->setConfig($feeds_importer->config[$type]['config']);
       }
     }
+    $errors = array_merge($errors, $importer->validateConfig());
+    form_error($form['importer'], theme('item_list', array('items' => $errors)));
   }
 
   $form_state['importer'] = $feeds_importer;
diff --git a/includes/FeedsConfigurable.inc b/includes/FeedsConfigurable.inc
index d809c20..19a4e06 100644
--- a/includes/FeedsConfigurable.inc
+++ b/includes/FeedsConfigurable.inc
@@ -173,6 +173,43 @@ abstract class FeedsConfigurable {
   }
 
   /**
+   * Gives the allowed values for each configuration key that allows a limit set
+   * of values.
+   *
+   * @return array
+   *   Per configuration key, the allowed values.
+   */
+  public function getConfigAllowedValues() {
+    return array();
+  }
+
+  /**
+   * Validates the configuration.
+   *
+   * @return array
+   *   A list of errors.
+   */
+  public function validateConfig() {
+    $config = $this->getConfig();
+    $errors = array();
+
+    // Check allowed values.
+    $allowed_values = $this->getConfigAllowedValues();
+    foreach ($allowed_values as $key => $values) {
+      if (isset($config[$key])) {
+        if (!in_array($config[$key], $values)) {
+          $errors[] = t('Invalid value %value for config option %key.', array(
+            '%value' => $config[$key],
+            '%key' => $key,
+          ));
+        }
+      }
+    }
+
+    return $errors;
+  }
+
+  /**
    * Returns whether or not the configurable has a config form.
    *
    * @return bool
diff --git a/includes/FeedsImporter.inc b/includes/FeedsImporter.inc
index 5286b1d..30591c9 100644
--- a/includes/FeedsImporter.inc
+++ b/includes/FeedsImporter.inc
@@ -177,6 +177,31 @@ class FeedsImporter extends FeedsConfigurable {
   }
 
   /**
+   * Validates the configuration.
+   */
+  public function validateConfig() {
+    $errors = parent::validateConfig();
+
+    // Validate errors of each plugin as well.
+    $plugin_types = array(
+      'fetcher' => t('Fetcher'),
+      'parser' => t('Parser'),
+      'processor' => t('Processor'),
+    );
+    foreach ($plugin_types as $type => $plugin_label) {
+      $plugin_errors = $this->$type->validateConfig();
+      foreach ($plugin_errors as $key => $error) {
+        $errors[] = t('@plugin: !error', array(
+          '@plugin' => $plugin_label,
+          '!error' => $error,
+        ));
+      }
+    }
+
+    return $errors;
+  }
+
+  /**
    * Return defaults for feed configuration.
    */
   public function configDefaults() {
diff --git a/plugins/FeedsProcessor.inc b/plugins/FeedsProcessor.inc
index 932b994..39820da 100644
--- a/plugins/FeedsProcessor.inc
+++ b/plugins/FeedsProcessor.inc
@@ -768,6 +768,19 @@ abstract class FeedsProcessor extends FeedsPlugin {
   }
 
   /**
+   * Overrides parent::getConfigAllowedValues().
+   */
+  public function getConfigAllowedValues() {
+    $allowed_values = parent::getConfigAllowedValues();
+
+    // Bundle.
+    $bundles = $this->bundleOptions();
+    $allowed_values['bundle'] = array_keys($bundles);
+
+    return $allowed_values;
+  }
+
+  /**
    * Overrides parent::configForm().
    */
   public function configForm(&$form_state) {
@@ -775,13 +788,19 @@ abstract class FeedsProcessor extends FeedsPlugin {
     $form = array();
 
     if (!empty($info['entity keys']['bundle'])) {
+      $default_bundle = $this->bundle();
       $form['bundle'] = array(
         '#type' => 'select',
         '#options' => $this->bundleOptions(),
         '#title' => !empty($info['bundle name']) ? $info['bundle name'] : t('Bundle'),
         '#required' => TRUE,
-        '#default_value' => $this->bundle(),
+        '#default_value' => $default_bundle,
       );
+      if (!isset($form['bundle']['#options'][$default_bundle])) {
+        $form['bundle']['#options'][$default_bundle] = t('Unknown bundle: @bundle', array(
+          '@bundle' => $default_bundle,
+        ));
+      }
     }
     else {
       $form['bundle'] = array(