sorry it's taken me so long to get back on this, but i've been very busy with other things.
let me give you an example.
last january we created a new domain
budgetwebsite.net. the zone file is the standard generated one. nothing was added or removed, and it worked like a charm. this morning however, i noticed the following line in ns2's syslog:
May 12 12:17:08 reaper named[1455]: zone budgetwebsite.net/IN: refresh: non-authoritative answer from master 83.149.67.24#53 (source 0.0.0.0#0)
mind that nothing has changed in the mean time (except for the increase in hosted domains). this (domains suddenly complaining about non-auth answers) will happen about once a week, but always with another domain. the only thing that resolves the situation is nuke the zone file (and it's entry in the bind's cfg) off ns2, remove the zone from DA, restart ns1 and ns2, recreate the zone on DA, recreate the slave entry on ns2, restart nd1 and ns2.
May 12 12:43:26 reaper named[1455]: zone budgetwebsite.net/IN: saved '/var/named/budgetwebsite.net.hosts' as '/var/named/db-7YPDDaWe'
May 12 12:43:26 reaper named[1455]: zone budgetwebsite.net/IN: Transfer started.
May 12 12:43:26 reaper named[1455]: transfer of 'budgetwebsite.net/IN' from 83.149.67.24#53: connected using 195.64.94.113#36147
May 12 12:43:26 reaper named[1455]: zone budgetwebsite.net/IN: transferred serial 2005051200
May 12 12:43:26 reaper named[1455]: transfer of 'budgetwebsite.net/IN' from 83.149.67.24#53: end of transfer
May 12 12:43:26 reaper named[1455]: zone budgetwebsite.net/IN: sending notifies (serial 2005051200)
now this isn't a really big problem cuz most of the zone files are the default generated ones, but we do have a few zone files that have considerably more entries.
(obviously, reaper is ns2)