Fascinating read!

After an exam, however, students often have difficulty remembering all of the things that they studied. Because you no longer have immediate use for it, the information sometimes feels like it has been flushed out of your memory.

Zeigarnik suggested that failing to complete a task creates underlying cognitive tension. This results in greater mental effort and rehearsal in order to keep the task at the forefront of awareness. Once completed, the mind is then able to let go of these efforts.

By thinking of uncompleted tasks often, we better remember them until they are complete.

Ah, so this is that nagging feeling of incompleteness.