LOCK TABLES takes a list of tables to lock. It may lock several 
  tables successfully and then encounter a tables that it can't lock, 
  e.g. because it's locked. In such case it needs to undo the locks on
  the already locked tables. And it does that. But it has also notified
  the relevant table storage engine handlers that they should lock.
  The only reliable way to ensure that the table handlers will give up
  their locks is to end the transaction. This is what UNLOCK TABLE 
  does : it ends the transaction if there were locked tables by LOCK 
  tables.
  It is possible to end the transaction when the lock fails in 
  LOCK TABLES because LOCK TABLES ends the transaction at its start 
  already. 
  Fixed by ending (again) the transaction when LOCK TABLES fails to
  lock a table.