Comment 1 for bug 1092344

This is an oldie, so some things have changed back and forth since.
Current behavior (v2.2.15) is to not exit when a slave is unreachable. This is desirable when you have a very long run (hours/days) with various slaves and at some point you want to take one offline (for whatever reason), but don't want tool to exit and lose all the work done.
I think the situation described in the report can be handled with --recursion-method=dsn , where you can tell the tool exactly which slaves to monitor, (using different user/passwords if necessary)

See:
https://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html#cmdoption-pt-online-schema-change--recursion-method