From time to time you may have to do some code review or log reviewing and something really unsafe may show up in your MySQL logs.  You may be trying to figure out why the  replication logs are growing so quickly, well one of the reasons may be due to a developer adding LIMIT x in one of their UPDATE statements.  Some developers may have the idea of well it works so I can use it right, well in the case of replication that would be a no.

From the MySQL documentation on why one should not use LIMIT in their UPDATE statements:

 

Statement-based replication of LIMIT clauses in DELETE, UPDATE, and INSERT … SELECT statements is unsafe since the order of the rows affected is not defined. (Such statements can be replicated correctly with statement-based replication only if they also contain an ORDER BY clause.) When such a statement is encountered:

 

When using STATEMENT mode, a warning that the statement is not safe for statement-based replication is now issued.

Currently, when using STATEMENT mode, warnings are issued for DML statements containing LIMIT even when they also have an ORDER BY clause (and so are made deterministic). This is a known issue. (Bug #42851)

When using MIXED mode, the statement is now automatically replicated using row-based mode.

 

If you see multiple warnings about the LIMIT being in an UPDATE statement please inform the maintainer(s) of the application’s SQL queries about the issue.  Not doing so can introduce problems down the road when using MySQL replication.