DHCP Subnet Mask Problem

On my 150+ machine network, I have two or three that get the wrong subnet mask when they renew their IP addresses via DHCP. (DHCP server running NT 4.0). Instead of renewing 192.168.0.X with a subnet mask of 255.255.255.0, they get the right addressrange, but a 255.255.0.0 mask. Since it only affects a few machines, I suspect it is a client problem, not a server problem. Anyone ever seen this before? Thanks!