When one clicks a button, or anything that can be clicked, the action that the button does will perform its function, without waiting for the mouse button to be released. It also doesn't matter how many clicks a button normally takes.

For example, if one were to click down on the "Options" button, the options pane will open, without the need for the mouse button to be released. Also, if one were to click down on a singleplayer level, the level will open up, making it impossible to delete or recreate a world, as it will leave the level select and enter gameplay.

This does not affect things like the creative inventory in-game, the game properly waits for a full click before switching block tabs (in the creative menu).