When items are picked up while the hotbar is full and while the creative inventory is open (the tab does not matter, so long as it is not the survival inventory), they will not enter the inventory.

Any items entering the inventory after failing to pick an item up will act as if the first slot is filled (where the item would normally be), and enter into the slot directly to the right of it, even though the first slot is empty.  Items can be directly put into the slot, and this seems to delete the item completely.

To replicate:

Go into creative mode and fill the hotbar.  Look straight down, open the creative inventory, grab an item from the creative inventory that won't stack into your hotbar, and drag it out of the inventory to drop it on the ground.  So long as the inventory is open on a creative tab when you pick up the item, it will not go into the inventory.