It is highly recommended to configure two or more DNS servers incase one is down.
I would count not configuring at least two as 'user error'. Many systems require you to enter a primary and alternate server in order to save a configuration.
The default setting on most computers seems to be: use the (wifi) router. I suppose telcos like that because it keeps the number of DNS requests down. So I wouldn't necessarily see it as user error.
The funny part with that is that sites like cloudflare say "Oh, yeah, just use 1.0.0.1 as your alternate", when, in reality, it should be an entirely different service.
