Here's a useful flag I found in the mysql manpage:
· --safe-updates, --i-am-a-dummy, -U Permit only those UPDATE and DELETE statements that specify which rows to modify by using key values. If you have set this option in an option file, you can override it by using --safe-updates on the command line. See the section called “MYSQL TIPS”, for more information about this option.
This should prevent you from making a mistake like this:
mysql> UPDATE tusers SET Fname = 'Oops', Lname = 'this-would-update-all-rows-in-tusers';
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column