This game features various item spots that have (usually) three different items that are possible, each with a set % chance to obtain a paticular item. Typically you would have to reload the game or wait enough in game time to try again in order to get what you want. If however, you have 99 of any of the items obtainable at that spot, it won't let you pick it up if that is what comes up. You can then try to pick it up again and it will recalculate your item. If you have 99 of two of the items possible, you can just jeep attempting to pick up the item until it finally comes up as the only other item. There are a few spots in the game where even the lesser items are too rare and it isn't feasible to have 99 of them but this trick should help you out immensely for 99% of these spots.