Error message: Invalid argument in taxonomy.module

Recently we had the following error message on the site:

warning: Invalid argument supplied for foreach in taxonomy.module...


So what's the problem. The node variables seems to be empty so
this fails. So the main question is way is this variable empty here.

After short debugging it was possible find out that the uid of the node table is not existant. This happened because the user table was changed. Once you have this node_load fails. This is quite a wtf.

To fix this you have to

• Find out the nodes without an author:
  
SELECT n.nid FROM {node} n LEFT JOIN {users} u ON u.uid = n.uid WHERE u.uid IS NULL;

• Set a author for this node:
  
UPDATE {node} SET uid = 3 WHERE nid = 123;


Once you have this the problem will go away.