Item spots usually have three different items that can appear, each with a set percentage chance to obtain a particular item. Normally you have to reload the game or wait enough in game time to try again to get the desired item. However, if you have 99 of any of the items obtainable at that spot, it will not let you collect that item if that is what appears. When you try to collect it, it will recalculate your item. If you have 99 of two of the possible items, you can just keep trying to collect the items until it finally drops the only other possible item. There are a few spots in the game where even the lesser items are too rare and it is not feasible to have 99 of them, but this trick should help for most spots in the game.