Scheduled tasks should _not_ be started before or during an upgrade

For example, this error can be thrown during a 1.4.4 to 2.0 upgrade:

2005-11-17 04:03:00,068 ERROR [sf.hibernate.util.JDBCExceptionReporter] Unknown column 'spacedescr0_.VERSIONCOMMENT' in 'field list'
2005-11-17 04:03:00,068 ERROR [sf.hibernate.util.JDBCExceptionReporter] Unknown column 'spacedescr0_.VERSIONCOMMENT' in 'field list'
2005-11-17 04:03:00,069 ERROR [sf.hibernate.util.JDBCExceptionReporter] Unknown column 'spacedescr0_.VERSIONCOMMENT' in 'field list'
2005-11-17 04:03:00,069 ERROR [sf.hibernate.util.JDBCExceptionReporter] Unknown column 'spacedescr0_.VERSIONCOMMENT' in 'field list'
2005-11-17 04:03:00,072 ERROR [sf.hibernate.util.JDBCExceptionReporter] could not load: com.atlassian.confluence.spaces.SpaceDescription#1
java.sql.SQLException: Unknown column 'spacedescr0_.VERSIONCOMMENT' in 'field list'
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2001)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1168)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1279)
at com.mysql.jdbc.Connection.execSQL(Connection.java:2281)
at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1634)
at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeQuery(NewProxyPreparedStatement.java:75)
at net.sf.hibernate.impl.BatcherImpl.getResultSet(BatcherImpl.java:87)
at net.sf.hibernate.loader.Loader.getResultSet(Loader.java:875)
at net.sf.hibernate.loader.Loader.doQuery(Loader.java:269)
at net.sf.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:133)
at net.sf.hibernate.loader.Loader.loadEntity(Loader.java:911)

If the scheduled job responsible for polling mail accounts is started before Confluence has had a chance to upgrade the CONTENT table.