dhcpack instead dhcpnak

[benek]

Hi,
I noticed a strange behavior of the dhcp server (the dhcp3-server 3.1.1-6 + 
lenny4 on debian). All users have so defined addresses:
"... the ethernet hardware aa:bb:cc:dd:ee:ff; fixed-address 10.20.30.40;..."
When I changing the ip address for a given mac in dhcpd.conf sometimes the 
server responds with DHCPACK to the DHCPREQUEST with a non-existent ip 
address and the user receives a non-existent settings in dhcpd.conf. 
Normally should respond DHCPNAK and force of a new lease process.
What could be causing this behavior and how I can prevent it.

Regards

benek