When you have two items and press on the slot of one and drop it, you will drop the original item, and if you pick it up, nothing gets added to your inventory. (This has only been done in creative mode)

Steps to Reproduce:

1. Have an emerald on your 1st slot and a diamond on the 2nd in an unmodded world.
2. Hold the diamond.
3. Click on 1 and Q at the same time.

Observed Results:

The player drops a diamond and has a diamond in his/her inventory
Expected Results:
The player drops an emerald and has a diamond in his/her inventory / The player drops a diamond and has an emerald in his/her inventory