[snuxoll]

> The quirk of the DNS is that if you have a CNAME record for a host name, you cannot have any other records for that same host. However, your top level domain example.com usually must have an NS and SOA record. Therefore, you cannot also add a CNAME record for example.com.

I discovered this when using a CNAME for a root-level domain and then wondering why I had spotty mail delivery. Turns out, quite a few mail systems and/or DNS resolvers handle this fine - but there are still quite a lot that don't.