If the networks start to overlap, they can come into conflict. Although it will not give you a "duplicate ip" error if both use DHCP and everything is configured properly, packages will stop arriving correctly.
For this to happen, the subnet has to allow for these ranges to overlap.
For example: 10.0.0.x with 255.255.255.0 and another one identical to that, even though the ip addresses handed out differ, will cause this.
This is also true for 10.0.x.x with a subnetmask of 255.255.0.0 which will overlap with 10.0.0.x with 255.255.255.0
But using 10.0.0.x with 255.255.255.0 and 10.0.100.x with 255.255.255.0 will work okay.
But a rule of thumb for any networking organisation: a 10.0.x.x range is a Class A network, which is only used when you expect to have a large network with vlan's etc. If you aim for a private home network, always stick wtih a Class C (192.168.x.x) network instead. This way, you can be ensured nothing will overlap and get in eachother's way.