pfSense

The DHCP pool automatic calculation on LAN done by the Setup Wizard always starts the pool at ".10" in relation to the given LAN IP and finishes it 10 addresses before the end of the subnet. This can result in:
1) The specified LAN IP could be in the DHCP pool
2) If someone uses an unusually small LAN subnet mask, then
a) ".10" might not be in the subnet
b) the subnet might not be big enough to make a DHCP range that goes from ".10" to 10 addresses from the end of the subnet. e.g. a LAN of /28 only has 16 addresses total.
3) A subnet mask of 32 can be selected - that just generally makes the world go wrong :)

Problem (1) is the most likely to happen - e.g. someone types in LAN IP 192.168.1.123 and it sits in the middle of the DHCP Pool.

I have some enhanced calculation and validation code for the setup wizard that I will submit...