Racker Hacker » MySQL: Errcode: 24 when using LOCK TABLES

There’s two ways to fix the problem. First, if you find that you only hit the limit during mysqldumps and never during normal database operation, just add --single-transaction to your mysqldump command line options. This will cause mysql to keep only one table open at a time.

However, if this happens while backups aren’t running, you may want to increase the open_files_limit in your MySQL configuration file. By default, the variable is set to 1,024 open files.

For further reading:
5.2.3. System Variables
7.13. mysqldump - A Database Backup Program