When "zabbix_server.conf" contains NodeID>0 (i.e. not standalone server) and database is not converted to DM mode the server still able to start and continue to work.
But server will not be able to operate properly.
In this case it's hard to figure out what's going on.

Similar case if DB already converted to the NodeID=2 (for instance) but config file contains NodeID=3
Server will start but it cannot operate properly.

How to avoid such cases:
Maybe server can check availability and matching of "configid" in the "config" table ?
For standalone server (NodeID=0) and its configid=1
For NodeID=2 its configid=200200000000001

If "configid" doesn't match then server has to stop work immediately with a detailed message clear for a user.