Jiva or Living Entity: Falldown of a Jiva

Q. I have read in many places about the phrase 'fall down'. I am a bit confused about what is the meaning of fall down of a devotee?

What does it actually mean when we say a devotee (both a neophyte and a elevated devotee) has fallen down? When can I consider that I have fallen down? Is it that when we deviate from the instructions of the spiritual master in a broader sense, or is it when we break any regulative principle; or is it both, or is it anything else? A Pure devotee cannot fall down (by Krishna's Grace). For aspiring devotees like us, since we are not pure, we are anyways fallen. So how can we identify that event of fall down  and take a recourse action to reinstate ourselves?