A huge problem are developers who lack a fundamental understanding of how the internet even works. I've had to explain how short, unqualified names resolve vs how fqdns resolve. Or why even you may not be able to reach another node in your proverbial cluster, because they are on different subnets. Or, why using GUIDs as hostnames is a generally bad idea, and will cause things to fail in unpredictable ways, especially with deeply nested subdomains.
Thunderbird and neomutt.