What happens if you have two ABR routers, each with the same area id on your network? For example,
routerA area-0 and area-1
routerB area-0 and area-1
I know it's not best practice. What problems would this cause? Thanks in advance.
I mostly agree with Edison that it makes little difference if you have 2 areas as area 1 and they are not connected. The one thing to be careful about which could be an issue: if the ABRs are configured to do summarization for area 1, then it could become ambiguous which area 1 a particular subnet belongs to. So it is possible that someone forwards a packet to routerA but the subnet destination is really reachable through routerB, but routerA would not know this.
You can partition non-zero areas without problem within OSPF. The problem is only with area 0.
The design above should work w/o problems as both areas will traverse the Area 0 and they will see each other as "O IA" instead of "O"