Supercell just announced another change coming on the Xmas update, this time affecting Magic Itens.
Today you can stack Magic Items from events beyond the Clan Castle capacity. You can have 6/5 Trining Potions for example, as long as the 6th was acquired from an event. This will change and the limit will be the enforced.
If you have free storage space for a specific Magic Item, then it will automatically be added to your Magic Item Storage once you complete a troop event;
If you do not have enough free space to store a particular Magic Item, first you'll have to free up some space, and then tap the "Claim" button from the event tracker. Those Magic Items will then be transferred to your Magic Item inventory, as well as the XP won from completing that challenge.