In the Reign of Giants DLC, the Backpack is a flammable item. When set on fire, everything insides won't be affected by fire but the Backpack will eventually burn to Ashes, dropping anything it had inside. Care should be taken when dropping a backpack near a campfire to prevent it from burning. The backpack is not waterproof, but items in the backpack will take longer to get wet than your main inventory. If the player does not have full water protection, it is recommended to keep one's fuel and food items in a backpack during the rain.
As of the Reign of Giants update, "A Feet of Strength", the Piggyback and its contents are waterproof and will not cause sanity drain while equipped. The Piggyback does not give the player any rain resistance, and using it in conjunction with an Umbrella will not protect the rest of the player's items from wetness. The movement speed penalty was also reduced from -20% to -10% in this update. Used with the Walking Cane, the penalty is negated and the player's speed increased to 115%.
Even though the importance of only two extra slots may be minimal, the Krampus Sack doesn't slow the player down like the Piggyback does, making it a much better alternative. However, with a lowly 1% drop rate and a significant time investment to spawn a Krampus, it should be said that during any given game it's possible that the player might spend more time trying to get the item than would be saved by not walking slower with the Piggyback. It may be better to ultimately treat the Krampus Sack as an unexpected bonus and/or something to only strive for in one's spare time.
The quickest way to spawn Krampus is by killing a Glommer, so players should take advantage of every full moon cycle to retrieve the Glommer's Flower and kill the Glommer in an area where any spawned Krampus will quickly die. Passive mobs will be aggressive towards Krampus, so a Pig or Bunnyman village would be an ideal location to kill a Glommer. Tooth Traps work as well, but Krampus might steal the traps instead of setting them off.
The Krampus Sack was added in the Naughty and Nice update. Before the Underground update, the Krampus Sack didn't appear as an icon on the map.
The Chef Pouch was added in the Eye of the Tiger Shark update to Shipwrecked. When the player begins the game as Warly, the Chef Pouch is actually dropped nearby, implying that it is supposed to go into the player's Inventory, but fails, as backpacks can not be held in the Inventory.
When Krampus wears the Krampus Sack, there are no chains surrounding the Sack, while the item obtained from killing Krampus has two chains draped across it. When worn, a faint jingling sound from the chains can be heard while the player is moving.
The Piggyback's name is a pun, on both the fact it is a backpack made from a pig's skin and on the action of giving a piggyback.
When Webber examines a backpack, he says that it is like a second abdomen that he can store stuff in. This most likely implies that Webber's inventory is actually the spider's abdomen.