In creative mode, if you have an item in hotbar slot 1 and hotbar slot 2, selecting item 2, going to your inventory and hovering over item 2 and pressing 1 will switch the items normally, but exiting the menu and right-clicking will cause the item to be replaced with whatever is in slot 1.
This bug was found in 12w40a, when switching items with number keys was added, and has been in every version since.