Using update.php on a wiki with replication has the potential of hitting a race condition where the master has finished an update operation, but the slave(s) hasn't; and then a query hits the slave but fails due to missing columns etc.
This can be shown more clearly by broken replication:
[20:37:15] <jcl> I'm having trouble upgrading MW from 1.20.8 to 1.23: during the update.php script, I'm getting a db query error: "select page_namespace, etc. from page where ..." from the function Title::newFromID is erroring with "Unknown column page_content_model" in 'field list' -- I've seen a few similar results from googling but nothing I've done so far has helped. Any suggestions?
[20:59:59] <jcl> well there you go, the slave threads aren't running, gr...
In a lot of cases, update.php may not be the correct tool for the job for updating these wikis, but it happens.
Maybe it should bail if more than 1 server. Or maybe more sensibly, queries to the slaves should be disabled.
To go along with this, some wfWaitForSlave() type calls should probably be added between updater operations too.