SQL sytax error when 'multiple values' selected on a node reference field.

Conditions:
The error is reported when I re-submit the field configuration.
De-selecting the 'Multiple Values' thingamy seems to avoid the error.

In this case, the offending node-reference field is used by two different content types. It's very strange but I can't replicate the problem with another node reference field shared by the same content types ... :(

user warning: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DISTINCT(node.nid) FROM node node 
LEFT JOIN content_field_equipment_required nod' 
at line 1 query: SELECT node_data_field_equipment_required.field_equipment_required_nid AS DISTINCT(node.nid) FROM node node 
LEFT JOIN content_field_equipment_required node_data_field_equipment_required ON node.vid = node_data_field_equipment_required.vid WHERE node.nid = 25 ORDER BY node_data_field_equipment_required.delta in /home/username/public_html/drupal5/includes/database.mysql.inc on line 172.

I'm baffled - any suggestions welcome.