Picking a Chest or Coffer as a Thief - The Thief job has the unique ability to pick a chest or coffer, without using the regular treasure keys from the zone. Instead, a crafted item is utilized that is used up in the process of trying to pick the chest or coffer. There are three of these tools that a Thief can choose to use, which seem to provide slightly different degrees of success: the Skeleton Key has the highest chance of success, the Living Key is less effective, and the Thief's Tools is the least successful. Using these items can only be done by a Thief main job, not when using a Thief as a subjob. Whichever item is chosen to try to pick the chest or coffer, it is used up in the attempt, whether the Thief succeeds or fails. If the Thief does succeed, he obtains the treasure just as if the chest or coffer was opened with a regular treasure key. When the attempt to pick a chest or coffer fails, the Thief can be in serious trouble. The potential hazards are different, depending on whether the Thief is trying to open a chest, or a coffer.
Chest Failures - If a Thief succeeds in picking a chest, the tool is used up and he obtains the treasure inside. An attempt to open a chest can simply fail, using up the tool. A critical failure when picking a chest also causes the Thief to be affected by a powerful weakness, which cannot be removed except by wearing off over time or by the death of the character. This weakness can last from 10 minutes up to an hour of real time. The weakness prevents the Thief from attempting to open any other chests or coffers, and acts as a powerful curse.
Coffer Failures - If a Thief succeeds in picking a coffer, the tool is used up and he obtains the treasure inside. An attempt to open a coffer can simply fail, using up the tool. A critical failure when picking a coffer also spawns a Mimic mob. These are monsters that look like a chest or coffer and are very difficult opponents, especially for a solo character. Mimics have a stun attack, a powerful poison, and use Draw-In on their opponents to keep them from running away. Only high level Thieves can safely solo a Mimic; most Thieves bring a Warp scroll when trying to pick a coffer, to get away if a Mimic appears.
The success rate for picking a chest or coffer varies widely, and is based partially on the tool used when trying to pick it. There are many theories about whether other things can change the success rate, whether they be certain stats (DEX, INT, etc.) or more esoteric variables such as the moon phase. What is known for certain is that the higher character level a Thief is, the better chance he will have in picking a chest or coffer. This is especially true in zones where the monsters are higher level; it is very difficult to pick a chest in a place where the average monster nearby is higher level than the Thief. The success rate varies from zone to zone with chests; there seem to be around five tiers of chests, with the higher tiers increasingly difficult to pick. This tier can be determined by the amount of the FFXI Gil  found inside, or by the level of the enemies in the zone; chests in lower level zones are much easier to pick than those in higher level zones. Coffers generally present the same difficulty when it comes to picking them; all coffer zones contain high level monsters. There is always some risk involved for a Thief trying to pick a chest or coffer. It can be worth the risk, though; a Thief who gets lucky can make a nice amount of FFXI Gil from opening chests and coffers in the higher level zones.