Previously, if you had a full inventory on your toon, but you pulled something out of a guild bank that was stackable, it would auto stack.
Example: Inventory is at 200 of 200 - you are entirely full. You have a partial stack of some green recipe, let's say Markarth Mead. A guildmate has put one in your guild bank, cluttering up the pool and you want to pull it out. Previously, even though you are 200 of 200 item slots, when you'd go to pull that single Markarth Mead recipe out, it would stack into your inventory.
Now, instead of being able to pull it into your inventory, you are given the "Your bags are full" message in the upper right corner of the screen. When you make room in your bags and pull the single Markarth Mead recipe out of the guild bank, it does not go into your partial stack. You must exit out of the gbank interface, open your bags, and hit the "stack items" prompt at the bottom of the screen.
PC-NA