As a ZigBee recognised test house we do a lot of certification testing of ZigBee products. One problem that we see repeatedly within the devices we test is the incorrect processing of the Default Response bit within the ZCL frame control header of an incoming frame.
The behaviour of a device which receives such a frame is defined in the ZigBee Cluster Library (075123r03) in section 220.127.116.11. This states that,
The default response command is generated when a device receives a unicast command, there is no other relevant response specified for the command, and either an error results or the Disable default response bit of its Frame control field is set to 0
What this basically means, is that when this bit is set to 0, then the receiving device must send back a Default Response frame if no other relevant response frame exists.