When you click on an item with your mouse and close the inventory, the item is not thrown out. Now, this in of itself is not really much of an issue, but the issue goes deeper; /clear also bugs with this. If you have an item held with your mouse and use /clear, the item that should have been cleared remains in your inventory

Steps to Recreate

1.) /give yourself any item

2.) Open your inventory and click on the item as if you're going to move it

3.) While the item is being held with your mouse, close the inventory

4.) Type /clear [item]

5.) Open your inventory and voila, you still have the item in your mouse

Again, this isn't some massive security flaw or anything massive, but it can be an inconvenience at times if someone knows this and uses it to their advantage