diff -ru commerce_price_table-1.1/commerce_price_table.install commerce_price_table/commerce_price_table.install
--- commerce_price_table-1.1/commerce_price_table.install	2012-05-04 09:54:12.000000000 +0200
+++ commerce_price_table/commerce_price_table.install	2013-01-24 20:42:29.950658485 +0100
@@ -45,3 +45,93 @@
     );
   }
 }
+
+/**
+ * This is not a real update, it just checks whether the price table
+ * entries as consistent, so the price calculations would not differ
+ * after removing the max_qty field.
+ */
+function commerce_price_table_update_7100() {
+  $fields = commerce_info_fields('commerce_price_table');
+  foreach ($fields as $fieldName => $field) {
+    if ($data_table = key($field['storage']['details']['sql']['FIELD_LOAD_CURRENT'])) {
+      // Select alls product ranges, ordered by min_qty
+      $result = db_select($data_table, 't')
+               ->fields('t', array('entity_type', 'entity_id', $fieldName.'_min_qty', $fieldName.'_max_qty'))
+               ->orderBy('t.entity_type, t.entity_id, t.'.$fieldName.'_min_qty', 'ASC')
+               ->condition('t.deleted', '0', '=')
+               ->execute();
+      $entity_type = null;
+      $entity_id = null;
+      $ranges = array();
+      while ($row = $result->fetchAssoc()) {
+        if ($entity_type != $row['entity_type'] || $entity_id != $row['entity_id']) {
+          if (!empty($ranges)) {
+              // next entity, evaluate
+              _commerce_price_table_validate_min_max_ranges($entity_type, $entity_id, $ranges);
+          }
+          $entity_type = $row['entity_type'];
+          $entity_id = $row['entity_id'];
+          $ranges = array();
+        }
+        $ranges[] = array($row[$fieldName.'_min_qty'], $row[$fieldName.'_max_qty']);
+      }
+      if (!empty($ranges)) {
+        // evaluate last sequence
+        _commerce_price_table_validate_min_max_ranges($entity_type, $entity_id, $ranges);
+      }
+    }
+  }
+}
+
+/**
+ * Ensures that the quantities of the given product are strictly monotone increasing,
+ * starting at 0 (1 is allowed, too) and ending at -1. Ranges must be an array of 
+ * min_qty, max_qty pairs, sorted by min_qty.
+ */
+function _commerce_price_table_validate_min_max_ranges($entity_type, $entity_id, $ranges) {
+  $result = true;
+  $qty = 0;
+  foreach ($ranges as $range) {
+      list($min, $max) = $range;
+      if ($qty == -1 || $min < 0) {
+          // Last element, or invalid value for min
+          $result = false;
+          break;
+      }
+
+      if ($max <= $min && $max != -1) {
+          // Max may only be smaller than min if it is the last element
+          $result = false;
+          break;
+      }
+
+      if ($qty == 0 && $min != 0 && $min != 1) {
+          // The sequence has to start at 0 or 1
+          $result = false;
+          break;
+      }
+
+      if ($min <= $qty) {
+          // The may not be two prices having the same min qty
+          $result = false;
+          break;
+      }
+
+      $qty = $max;
+  }
+
+  if ($qty > 0) {
+      // Missing last element -1
+      $result = false;
+  }
+
+  if (!$result) {
+    if ($entity = reset(entity_load($entity_type, array($entity_id)))) {
+      if ($uri = entity_uri($entity_type, $entity)) {
+        drupal_set_message('non monotonically increasing price table quantity range in product '
+            . l($entity->title, $uri['path']) . '. Calculated prices may differ.', 'warning');
+      }
+    }
+  }
+}
diff -ru commerce_price_table-1.1/commerce_price_table.module commerce_price_table/commerce_price_table.module
--- commerce_price_table-1.1/commerce_price_table.module	2012-05-04 09:54:12.000000000 +0200
+++ commerce_price_table/commerce_price_table.module	2013-01-25 10:06:40.506493398 +0100
@@ -190,24 +190,94 @@
 /**
  * Implements hook_field_validate().
  */
-function commerce_price_table_field_validate($entity_type, $entity, $field, $instance, $langcode, &$items, &$errors) {
-  // Ensure only numeric values are entered in price fields.
+function commerce_price_table_field_validate($entity_type, $entity, $field, $instance, $langcode, $items, &$errors) {
+  $lower_bound = 0;
+  $infinite = false;
+  $incomplete = false;
+  uasort($items, 'commerce_price_table_sort_by_qty');
   foreach ($items as $delta => $item) {
-    if (!empty($item['amount']) && !is_numeric($item['amount'])) {
+    if (empty($item['amount'])) {
+      continue;
+    }
+
+    // Ensure only numeric values are entered in price fields.
+    if (!is_numeric($item['amount'])) {
       $errors[$field['field_name']][$langcode][$delta][] = array(
         'error' => 'price_numeric',
         'message' => t('%name: you must enter a numeric value for the price.', array('%name' => check_plain($instance['label']))),
       );
     }
 
+    // min_qty may never be less than zero, and max_qty may never be less then min_qty (exception: -1)
+    if ($item['min_qty'] < 0) {
+      $errors[$field['field_name']][$langcode][$delta][] = array(
+        'error' => 'price_table_quantity',
+        'message' => t('%name: Min quantity must be greater or equal to zero.', array('%name' => check_plain($instance['label']))),
+      );
+    }
+
     if ($item['max_qty'] < $item['min_qty'] && $item['max_qty'] <> -1) {
       $errors[$field['field_name']][$langcode][$delta][] = array(
         'error' => 'price_table_quantity',
-        'message' => t('%name: Max quantity needs to be higher than min quantity.', array('%name' => check_plain($instance['label']))),
+        'message' => t('%name: Max quantity needs to be higher or same than min quantity.', array('%name' => check_plain($instance['label']))),
       );
     }
 
-    // @TODO Add extra validations, as no repeating qty and always force to have quantity for 1?.
+    if ($item['max_qty'] == -1) {
+      // There may only be a single infinite item.
+      if ($infinite) {
+        $errors[$field['field_name']][$langcode][$delta][] = array(
+          'error' => 'price_table_quantity',
+          'message' => t('%name: Only one infinite may quantity is allowed.', array('%name' => check_plain($instance['label']))),
+        );
+      }
+      $infinite = true;
+    }
+
+    // Ensure that there are no overlapping intervals
+    if ($item['min_qty'] < $lower_bound) {
+      $errors[$field['field_name']][$langcode][$delta][] = array(
+        'error' => 'price_table_quantity',
+        'message' => t('%name: Overlapping intervals.', array('%name' => check_plain($instance['label']))),
+      );
+    }
+
+    // And finally, check for gaps
+    if ($item['min_qty'] > $lower_bound) {
+      // Exception: first item may start with 1.
+      if ($item['min_qty'] != 1 || $lower_bound != 0) {
+        $incomplete = true;
+      }
+    }
+
+    // If max_qty is -1, this should be the last iteration
+    $lower_bound = $item['max_qty'] + 1;
+
+    // @TODO Always force to have quantity for 1?
+  }
+
+  // Check if incomplete intervals are allowed or not
+  if ($incomplete || !$infinite) {
+    // Determine bundle
+    $bundle = 'product';
+    $info = entity_get_info($entity_type);
+    if (!empty($info['entity keys']['bundle'])) {
+      if (isset($entity->{$info['entity keys']['bundle']})) {
+        $bundle = $entity->{$info['entity keys']['bundle']};
+      }
+    }
+
+    foreach (commerce_price_table_get_field_instance_settings($entity_type, $bundle) as $setting) {
+      if (isset($setting['settings']['commerce_price_table']['require_complete_intervals'])) {
+        if ($setting['settings']['commerce_price_table']['require_complete_intervals'] == TRUE) {
+          $errors[$field['field_name']][$langcode][0][] = array(
+            'error' => 'price_table_quantity',
+            'message' => t('%name: Price table intervals are incomplete.', array('%name' => check_plain($instance['label']))),
+          );
+        }
+        break;
+      }
+    }
   }
 }
 
@@ -263,6 +333,12 @@
     '#description' => t('Activate this checkbox to hide the default price and display the price table instead.'),
     '#default_value' => isset($settings['commerce_price_table']['hide_default_price']) ? $settings['commerce_price_table']['hide_default_price'] : NULL,
   );
+  $form['commerce_price_table']['require_complete_intervals'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Require complete intervals'),
+    '#description' => t('Activate this checkbox to disallow gaps between price table intervals.'),
+    '#default_value' => isset($settings['commerce_price_table']['require_complete_intervals']) ? $settings['commerce_price_table']['require_complete_intervals'] : NULL,
+  );
   return $form;
 }
 
@@ -466,22 +542,47 @@
   }
 
   // Sort the items by quantity and return the matching one.
+  $nearest = null;
+  $nearest_distance = PHP_INT_MAX;
+  $fallback = false;
   uasort($items, 'commerce_price_table_sort_by_qty');
   foreach ($items as $item) {
     if ($quantity <= $item['max_qty'] && $quantity >= $item['min_qty']) {
       return $item;
     }
+
+    if ($quantity < $item['min_qty']) {
+      // We have a gap. Fall back to default price.
+      $fallback = true;
+    }
+
+    // Calculate distance for fallback
+    $distance = abs($quantity - $item['min_qty']);
+    if ($item['max_qty'] > 0) {
+      $distance = min($distance, abs($quantity - $item['max_qty']));
+      if ($distance < $nearest_distance) {
+        $nearest = $item;
+        $nearest_distance = $distance;
+      }
+    }
   }
 
-  // Handle the unlimited qty.
-  foreach ($items as $item) {
-    if ($item['max_qty'] == -1) {
-      return $item;
+  if (!$fallback) {
+    // Handle the unlimited qty.
+    foreach ($items as $item) {
+      if ($item['max_qty'] == -1) {
+        return $item;
+      }
     }
   }
 
-  // We fallback to the higher one if no match was found.
-  return end($items);
+  // Fallback to default price (or nearest item as a last resort,
+  // which might be meaningful if the default price has been hidden).
+  if (isset($product->commerce_price[LANGUAGE_NONE][0]) && !empty($product->commerce_price[LANGUAGE_NONE][0]['amount'])) {
+    return $product->commerce_price[LANGUAGE_NONE][0];
+  } else {
+    return $nearest;
+  }
 }
 
 /**
@@ -490,9 +591,25 @@
 function commerce_price_table_sort_by_qty($a, $b) {
   $a_qty = (is_array($a) && isset($a['min_qty'])) ? $a['min_qty'] : 0;
   $b_qty = (is_array($b) && isset($b['min_qty'])) ? $b['min_qty'] : 0;
+
   if ($a_qty == $b_qty) {
-    return 0;
+    // Sort by max_qty
+    $a_qty = (is_array($a) && isset($a['max_qty'])) ? $a['max_qty'] : 0;
+    $b_qty = (is_array($b) && isset($b['max_qty'])) ? $b['max_qty'] : 0;
+    if ($a_qty == $b_qty) {
+        return 0;
+    }
+
+    // Special cases: -1 means infinite
+    if ($a_qty == -1) {
+        return 1;
+    }
+
+    if ($b_qty == 1) {
+        return -1;
+    }
   }
+
   return ($a_qty < $b_qty) ? -1 : 1;
 }