Set up a template containing a full DNS zone definition (SOA, A records, etc.)
Create a virtserver (A) and enable "create DNS zone" (such as example.com)
Create a subdomain-server (B) under virtserver (A) and tell it to use A's DNS zone (such as placing foo.example.com's records inside the example.com zone)
Bam - clash. Complains that there is already a SOA record in the zone.
Solution: When adding a domain to an existing zone, skip the SOA record definition, $TTL, and so on.
Btw, I am not advocating merging zones (I don't do it myself, it's too messy), but I discovered this bug and reported it since SOMEONE out there may want to merge zones.