Assume a local business has a domain 'taco.rentals'. They setup a pi-hole with some upstream DNS servers. For this example, lets say the upstream point to their local DNS server and OpenDNS for additional filtering. Every so often when the business tries to resolve local services such as ad.taco.rentals or fileserver.taco.rentals. These local domain names resolve about 70% of the time because some of the queries to the local DNS sever while others go to the OpenDNS server. You can set up strict-order to work manually editing the config files, but it would be a nice to have the feature supported in the GUI.
On a side note there are a lot of taco domains that are not claimed yet. My favorite so far: