Like the title says, when you shift click items in a chest, they'll fill up your main inventory, but won't go in your backpack even when there's empty slots. This makes it quite a bit more difficult to move around the content of large amounts of chests.
1. open a chest whilst wearing a backpack.
2. shift click items in the chest until your main inventory is filled up.
3. continu shift clicking items with full main inventory but with empty slots in your backpack.
4. notice that the game doesn't consider your backpack as inventory.