When in creative mode if you use numbers to switch an item with an item in that is in your hand (and this only works for switching blocks that are already in the hotbar with the block in your hand) the item will not register it has been switched and revert to what was previously in your hand.

The first picture shows the starting position (with just 2 arbitrarily chosen blocks)
second shows the 2 items switched after hovering the mouse over the orange and pressing the #1 on my keyboard
third shows after what you think would be placing the orange wool but it got replaced with what was previously in my hand and now there are two green wool in the hotbar (nothing else was in inventory)

this also only works if you have a creative tab open and not the inventory tab open when switching blocks.