Subnet Mask “Slippage”

What could cause the subnet mask of a Wintel machine on a large network to “Slip” its subnet mask bits?
The machines in question will be assigned an IP address from the Win2K DHCP Server and work fine for a while but then the Subnet mask, nothing else, will “slip” from a 24 bit to an 8 bit Mask. It’s happenned on various machines. It hasn’t happened on our Macs or Cisco stuff. It only happens to Clients of the DHCP server and then, not all of them.