Incorrect key file for table '/tmp/

Disk space can be low.

Try to tune your mysql with mysqltuner: http://rackerhacker.com/mysqltuner/

Example mysql.cnf configuration:

max_allowed_packet = 256M
key_buffer_size=256M
key_buffer=258M
innodb_buffer_pool_size = 100M
query_cache_type=1
query_cache_limit=64M
query_cache_size=64M
join_buffer_size = 2M
tmp_table_size = 512M
max_heap_table_size = 512M
thread_cache_size = 4
table_cache = 128
log-slow-queries = /var/lib/mysql/slow_query.log
log-error=/var/lib/mysql/mysqld.log

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This error usually occurs when you got a corrupt table. The best way to solve this is to perform repair. This article might help:

http://dev.mysql.com/doc/refman/5.0/en/repair-table.html