When setting up local networks people commonly use a top level domain like 'my.lan', 'my.network', 'my.local'. Instead of using one of these non-reserved domains that may one day end up as a TLD, it is recommended to use 'my.internal'.
If the 'private' TLD you're using suddenly becomes real, then you can ship off data, every possibly unencrypted data and connection requests to computers you do not control.