I've seen this bug since 1.7, and I feel like it's about time to report it.

This is VERY challenging to describe, so read these steps CAREFULLY.
Once you do it right you'll understand my plight in putting the bug into words.

Here's how to reproduce it -
1 Open up a Minecraft world
2 Go into creative mode
3 Press "e" to open the creative inventory, keep it open as well.
4 Put one block in your first slot and another, different in your second slot (it doesn't have to be your second but for the sake of this explanation it's just easier.)
5 Position your mouse over of the blocks in the hotbar, as if to pick it up, but do not click.
6 Press the number key of the other block (for example, if you have cobble in slot 1 and grass in slot 2, mouse over the grass and press 1)
7 You SHOULD have seen the blocks switched in the hotbar, this is not a bug, but a functional feature of the game.
8 Close the creative menu.
9 Scroll with the mouse wheel to the block that you had previously moused over, even though it's been swapped to the other slot.
10 Attempt to place it
11 If you could follow my mess of an explanation, you've just magically placed a block of one ID using a block of another!