Here's another suggestion that doesn't explain your problem, but that
probably helps you solve it by doing things slightly differently:

SELECT P.ID, P.USERID, U.USERNAME, P.MSG, P.POSTED,
P.LAST_EDITED, P.LAST_EDITED_BY, U1.USERNAME, P.IPADDRESS, P.TOPICID,
U.SHOW_SIG, U.SIGNATURE
FROM DNFBB_POSTS P
LEFT JOIN DNFBB_USERS U ON (U.ID = P.USERID)
LEFT JOIN DNFBB_USERS U1 ON (U1.ID = P.LAST_EDITED_BY)
WHERE (P.TOPICID = 1) AND
EXISTS(SELECT *
FROM DNFBB_TOPICS T
JOIN DNFBB_FORUMS F ON (F.ID = T.FORUMID)
JOIN DNFBB_USERGROUPSTOFORUM UGUF ON (UGUF.FORUM_ID = F.ID)
JOIN DNFBB_USERTOUSERGROUPS UTUG
ON (UTUG.USERGROUP_ID = UGUF.USERGROUP_ID)
WHERE T.ID = P.TOPICID AND UTUG.USER_ID = 1);

i.e. no DISTINCT, but move the part that I presume made DISTINCT
necessary - even though it isn't in any output column - into a simple
EXISTS part.

Though this only helps if my assumptions that the selected column with
this statement is DISTINCT by nature. And, I guess I'm not the only
person still curious why your initial statement failed.

HTH,
Set

--- In firebird-support@yahoogroups.com, "Si Carter" wrote:
> > If there are no NULLS or users that aren't in P.USERID or P.
> > LAST_EDITED_BY, you may even avoid LEFT JOINs altogether, and if
> > not an alternative would be to change to subselects for those two
> > USERNAME-fields.
>
> Your right there, I missed the boat on that one. I will change the
> sql and test it.
>
> FWIW, the distinct still raised the existing exception though :-(