Unsupported operand types if a field type is missing.

Please make sure that you run on latest versions of both modules/code.

drupal-7.0-rc2

You also need latest Drupal HEAD.

I can cause it, too.
Fatal error: Unsupported operand types in sites\all\modules\entity\modules\field.info.inc on line 20

It seems to be happening AFTER the node is saved.
I have a feeling this is triggered by upgrading the Token module.

My order of events

1. Setup Drupal7
2. Create various content-types
3. Setup Ubercart for Drupal 7
4. Add a product class
5. Update the product class (node type) to have several required fields
6. save the content type
7. Create a new node of that content-type
8. Fill in required fields
9. Press Save & continue
10. KABLOOIE: See the fatal error.

I'm willing to bet on #5's explanation, too:
field_info_field_types($field['type']) looks as if it should return a BOOL.
If this is the case, bool + array cannot work.
If it is returning an array array + array claims to be PHP4 and 5 compatible. (see: http://us3.php.net/manual/en/language.operators.array.php ).

I have a strong feeling array + array was an undocumented improvement to early versions of PHP5. It may look wrong, but it is actually a form of array_merge. Nonetheless, this error shouldn't occur, since I'm using the most recent PHP5 version anyway.

Read up on the field_info_field_types in Core. Looks like if a field type is provided, a STRING is returned; which WILL cause the expression identified to FAIL in PHP.

This can be overcome by putting the output of field_info_field_types into an array with a key that needs to be placed in the other side of the expression; OR ... improving the naming of hooks so they're not as easily misused.

See: CORE/modules/field/field.info.inc @ line 471

/**
 * Returns information about field types from hook_field_info().
 *
 * @param $field_type
 *   (optional) A field type name. If ommitted, all field types will be
 *   returned.
 *
 * @return
 *   Either a field type description, as provided by hook_field_info(), or an
 *   array of all existing field types, keyed by field type name.
 */
function field_info_field_types($field_type = NULL) {
  $info = _field_info_collate_types();
  $field_types = $info['field types'];
  if ($field_type) {
    if (isset($field_types[$field_type])) {
      return $field_types[$field_type];
    }
  }
  else {
    return $field_types;
  }
}

It returns the field type info array, thus the array+array operand is fine. But maybe the field-type in question is not available any more as the module is deactivated, thus the error is triggered?

The attached patch should fix it. Please test.

At least the additional check won't harm, so I've just committed #9.