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• saving invalid entity such as:

  • missing type from the node object (e.g. wrongly generated by Devel Generate?);

• orphaned nodes in node table;

• invalid rows in field_data_field_xxx table;

• having orphaned nodes, caused by:

  • search view having indexed nodes which aren't in the system;
  • other modules (like Feeds) removing nodes from Drupal improperly;

• nodes having invalid/non-existing references; for term data this can be checked by:

    drush sqlq "SELECT nid, title from node WHERE nid IN (select entity_id from  field_data_field_some_ref WHERE field_some_ref_tid not IN (select tid from taxonomy_term_data));"
  

  Where field_data_field_some_ref is data field of field_some_ref which is type of 'Term reference'.

  See: DB records not deleted for Term Reference Fields after Term is Deleted

• having orphaned taxonomy terms (so you may use Taxonomy Orphanage);

• entity caching issue;

• invalid custom code (human error) such as:

  • badly implemented Entity API,
  • badly implemented CTools/Form API,
  • !empty() test on the entity variable before calling field_get_items(),
  • rendering field by invalid calls,
  • triggering functions which shouldn't be triggered on certain pages (e.g. field_view_field()),

• saving invalid entity such as:

  • missing type from the node object (e.g. wrongly generated by Devel Generate?);

• orphaned nodes in node table;

• invalid rows in field_data_field_xxx table;

• having orphaned nodes, caused by:

  • search view having indexed nodes which aren't in the system;
  • other modules (like Feeds) removing nodes from Drupal improperly;

• nodes having invalid/non-existing references; for term data this can be checked by:

    drush sqlq "SELECT nid, title from node WHERE nid IN (select entity_id from  field_data_field_some_ref WHERE field_some_ref_tid not IN (select tid from taxonomy_term_data));"
  

  Where field_data_field_some_ref is data field of field_some_ref which is type of 'Term reference'.

  See: DB records not deleted for Term Reference Fields after Term is Deleted

• having orphaned taxonomy terms (so you may use Taxonomy Orphanage);

• entity caching issue;

• when using Rules Link module;

• invalid custom code (human error) such as:

  • badly implemented Entity API,
  • badly implemented CTools/Form API,
  • !empty() test on the entity variable before calling field_get_items(),
  • rendering field by invalid calls,
  • triggering functions which shouldn't be triggered on certain pages (e.g. field_view_field()),

• saving invalid entity such as:

  • missing type from the node object (e.g. wrongly generated by Devel Generate?);

• orphaned nodes in node table;

• invalid rows in field_data_field_xxx table;

• having orphaned nodes, caused by:

  • search view having indexed nodes which aren't in the system;
  • other modules (like Feeds) removing nodes from Drupal improperly;

• nodes having invalid/non-existing references; for term data this can be checked by:

    drush sqlq "SELECT nid, title from node WHERE nid IN (select entity_id from  field_data_field_some_ref WHERE field_some_ref_tid not IN (select tid from taxonomy_term_data));"
  

  Where field_data_field_some_ref is data field of field_some_ref which is type of 'Term reference'.

  See: DB records not deleted for Term Reference Fields after Term is Deleted

• having orphaned taxonomy terms (so you may use Taxonomy Orphanage);

• entity caching issue;

• invalid custom code (human error) such as:

  • badly implemented Entity API,
  • badly implemented CTools/Form API,
  • !empty() test on the entity variable before calling field_get_items(),
  • rendering field by invalid calls,
  • triggering functions which shouldn't be triggered on certain pages (e.g. field_view_field()),

If this is happening on node save, check this EntityMalformedException post at SO for more detailed instructions.

• saving invalid entity such as:

  • missing type from the node object (e.g. wrongly generated by Devel Generate?);

• orphaned nodes in node table;

• invalid rows in field_data_field_xxx table;

• having orphaned nodes, caused by:

  • search view having indexed nodes which aren't in the system;
  • other modules (like Feeds) removing nodes from Drupal improperly;

• nodes having invalid/non-existing references; for term data this can be checked by:

    drush sqlq "SELECT nid, title from node WHERE nid IN (select entity_id from  field_data_field_some_ref WHERE field_some_ref_tid not IN (select tid from taxonomy_term_data));"
  

  Where field_data_field_some_ref is data field of field_some_ref which is type of 'Term reference'.

  See: DB records not deleted for Term Reference Fields after Term is Deleted

• having orphaned taxonomy terms (so you may use Taxonomy Orphanage);

• entity caching issue;

• invalid custom code (human error) such as:

  • badly implemented Entity API,
  • badly implemented CTools/Form API,
  • !empty() test on the entity variable before calling field_get_items(),
  • rendering field by invalid calls,
  • triggering functions which shouldn't be triggered on certain pages (e.g. field_view_field()),

If this is happening on node save, check this EntityMalformedException post at SO for more detailed instructions.

• saving invalid entity such as:

  • missing type from the node object (e.g. wrongly generated by Devel Generate?);

• orphaned nodes in node table;

• invalid rows in field_data_field_xxx table;

• having orphaned nodes, caused by:

  • search view having indexed nodes which aren't in the system;
  • other modules (like Feeds) removing nodes from Drupal improperly;

• nodes having invalid/non-existing references; for term data this can be checked by:

    drush sqlq "SELECT nid, title from node WHERE nid IN (select entity_id from  field_data_field_some_ref WHERE field_some_ref_tid not IN (select tid from taxonomy_term_data));"
  

  Where field_data_field_some_ref is data field of field_some_ref which is type of 'Term reference'.

  See: DB records not deleted for Term Reference Fields after Term is Deleted

• having orphaned taxonomy terms (so you may use Taxonomy Orphanage);

• entity caching issue;

• invalid custom code (human error) such as:

  • badly implemented Entity API,
  • badly implemented CTools/Form API,
  • !empty() test on the entity variable before calling field_get_items(),
  • rendering field by invalid calls,
  • triggering functions which shouldn't be triggered on certain pages (e.g. field_view_field()),

^{Note: Using dd() function from Devel will generate the debugging info to the file in your Drupal temp folder (temporary://drupal_debug.txt) with backtrace dump, otherwise could be too big and difficult to read when dumping on the screen. When using var_dump(), it's easier to call die(); after the call and check the dump in the view-source mode of the page.}

• saving invalid entity such as:

  • missing type from the node object (e.g. wrongly generated by Devel Generate?);

• orphaned nodes in node table;

• invalid rows in field_data_field_xxx table;

• having orphaned nodes, caused by:

  • search view having indexed nodes which aren't in the system;
  • other modules (like Feeds) removing nodes from Drupal improperly;

• nodes having invalid/non-existing references; for term data this can be checked by:

    drush sqlq "SELECT nid, title from node WHERE nid IN (select entity_id from  field_data_field_some_ref WHERE field_some_ref_tid not IN (select tid from taxonomy_term_data));"
  

  Where field_data_field_some_ref is data field of field_some_ref which is type of 'Term reference'.

  See: DB records not deleted for Term Reference Fields after Term is Deleted

• having orphaned taxonomy terms (so you may use Taxonomy Orphanage);

• entity caching issue;

• invalid custom code (human error) such as:

  • badly implemented Entity API,
  • badly implemented CTools/Form API,
  • !empty() test on the entity variable before calling field_get_items(),
  • rendering field by invalid calls,
  • triggering functions which shouldn't be triggered on certain pages (e.g. field_view_field()),

^{Note: Using dd() function from Devel will generate the debugging info to the file in your Drupal temp folder (temporary://drupal_debug.txt) with backtrace dump, otherwise could be too big and difficult to read when dumping on the screen. When using var_dump(), it's easier to call die(); after the call and check the dump in the view-source mode of the page.}