Practice Lock

Practice Lock was a poor type of lockbox usually made by Engineers that could be used to practice Lockpicking. This was a slow and potentially expensive way to increase your Lockpicking skill.

Information

The Practice Lock can only be used once. After that, it is opened and completely worthless. Many players practiced with renewable lockboxes off the Merchant Coast near Ratchet, Amani Catacombs in Ghostlands or Alther's Mill in Redridge Mountains.
On September 2, 2007, Blizzard patched-in support for the auction house telling potential buyers whether practice locks have been opened or not. Previous to this, unscrupulous players could auction their used practice locks to be bought by unsuspecting players.
The Practice Lock is the only item categorized as "Junk" in the Engineering profession-window.
Interestingly enough, Practice Locks do have a practical use. They have a skillup-potential of 100 → 115 → 122 → 130, which means making them can be used to level Engineering.