MYSQLDUMP --skip-lock-tables
When performing a backup of a database, MySQL locks the tables by default in order to ensure that the data does not change while the backup is under way. Instructing MySQL to skip this process opens the data to the possibility that some user will insert or delete or do some other operation that would result in an incomplete backup. Performing a normal query should not be a problem since data is not being changed.

You might want to try an insert or delete operation while a backup is under way to test more fully. The type of database ISAM/Innodb is also an issue since the behaviour may be different.

See for example MySQL docs and a similar question here.