cron: Invalid argument supplied for foreach() in node.module line 521

I'm starting a new thread here for an issue that was erroneously appended to an old unrelated issue. Two users have independently reported it:

• http://drupal.org/node/171539#comment-313132 — D52, line 521
• http://drupal.org/node/171539#comment-314664 — D51, line 504

The foreach() error is followed by two aftereffects:

• implode() [function.implode]: Bad arguments
• SQL Syntax error in
  SELECT n.nid, n.vid, n.type, n.status, n.created, n.changed, n.comment, n.promote, n.sticky, r.timestamp AS revision_timestamp, r.title, r.body, r.teaser, r.log, r.format, u.uid, u.name, u.picture, u.data FROM node n INNER JOIN users u ON u.uid = n.uid INNER JOIN node_revisions r ON r.vid = n.vid WHERE
  (note the empty WHERE clause)

It's this code inside node_load(), node.module, called from cron:

    // Turn the conditions into a query.
    foreach ($param as $key => $value) {
      $cond[] = 'n.'. db_escape_string($key) ." = '%s'";
      $arguments[] = $value;
    }
    $cond = implode(' AND ', $cond);

The SQL-errors even remains during cron even after deleting all comments and content on the site.

The errors during cron disappear if i disable the subsriptions module. But initially after the first enabling of the subscription module there were no errors. There start a few days ago.

This is nasty! I don't know how to reproduce it here — any ideas on how to tackle this, chx?