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In the knapsack problem, we are given a knapsack of some capacity and a set of items, each with a size and a value. The goal is to pack a selection of these items fitting the knapsack that maximizes the total value. The online version of this problem reveals the items one by one. For each item, the algorithm must decide immediately whether to pack it or not. We consider a natural variant of this problem, coined removable online knapsack. It differs from the classical variant by allowing the removal of packed items. Repacking is impossible, however: Once an item is removed, it is gone for good. We analyze the advice complexity of this problem. It measures how many advice bits an omniscient oracle needs to provide for an online algorithm to reach any given competitive ratio, which is, understood in its strict sense, just the approximation factor. We show that the competitive ratio jumps from unbounded without advice to near-optimal with just constantly many advice bits, a behavior unique among all problems examined so far. We also examine algorithms with barely any advice, for example just a single bit, and analyze the special case of the proportional knapsack problem, where an item's size always equals its value. We show that advice algorithms have various concrete applications and that lower bounds on the advice complexity of any problem are exceptionally strong. Our results improve some of the best known lower bounds on the competitive ratio for randomized algorithms and even for deterministic deterministic algorithms in established models such as knapsack with a resource buffer and various problems with multiple knapsacks. The seminal paper introducing knapsack with removability proposed such a problem for which we can even establish a one-to-one correspondence with the advice model; this paper therefore also provides a comprehensive analysis for this neglected problem.