Error 200 whilst updating table

Hi again!
I have found a very strange error.
I have  middle size table called SALES - about 50 columns, 5000 rows which
has triggers defined on it - they insert information about all changes in
SALES into
another (let's say log-history) table.
I was trying to update the SALES table with the values from another table
(called ORGANIZATIONS) using rather simple, correlated subquery.
I was little confused when I received an error 200 - Duplicate Key.
I checked all the contrainst and key's definition but there is no reason for
that.
As it was very important to me I started to look for the workaround.
And I have found, that I can use this query and it worked very well when I
limited
the updated range to about 50 rows. So I updated next 50 rows, then 50
another and so on...
And this way I have updated all the table? tiring, isn't it? ;))
And when I finished it I executed the original query - without record
limitation and it executed succesfully...
What is more funny - I tried to reproduce this behaviour using another
table, but I couldn't -
everything went perfectly.
Any ideas?

Best regards,
Marcin Pytel