It is possible for the data on the master and slave to become different if a statement is designed in such a way that the data modification is nondeterministic; that is, left up the query optimizer. (This is in general not a good practice, even outside of replication.) Examples of nondeterministic statements include DELETE or UPDATE statements that use LIMIT with no ORDER BY clause; see Section 16.3.1.12, “Replication and LIMIT”, for a detailed discussion of these.