The last time I set up DNS at home, I decided to use a fictitious and undelegated subdomain under my ISP's domain name. This structure did not create any "extra" problems in the short term.
But I suppose that I still ran the risk that the subdomain could "become real", or draw attention from security admins, or I would change ISPs.
How? I’m not saying this is great practice — there are certainly better options — but no one outside your network will ever know about it. It also won’t matter if you switch ISPs.
It's not a problem (well, most of the time), but you would see the requests for 'internal' resources in DNS (ie your machine is not on your network but tries to resolve the internal DNS records) and in certificate checks even for non-public PKIs
But I suppose that I still ran the risk that the subdomain could "become real", or draw attention from security admins, or I would change ISPs.