MySQL 5.1 introduces row based binary logging.  In fact, the default binary logging format in GA versions of MySQL 5.1 is 'MIXED' STATEMENT*;   The binlog_format  variable can still be changed per sessions which means it is possible that some of your binary log entries will be written in a row-based fashion instead of the actual statement which changed data, even when the global setting on the master is to write binary logs in statement mode.   The row-based format does offer advantages particularly if triggers or stored procedures are used, or if non deterministic functions like RAND() are used in DML statements.

A statement based replication slave can get out of sync with the master fairly easily, especially if data is changed on the slave.   It is possible for a statement to execute successfully on a slave even if the data is not 100% in sync, so MySQL doesn't know anything is wrong.  This isn't the case …