Glitch with paper 2D sprite

I’m making a 2D game and have a problem with player character sprite. When it is moving, a small line appeaars from time to time near its back (on the very edge of the sprite). It can appear or disappear while player is moving, What can be the reason and how to get rid of it? (in this picture it is to the left of the sprite)

302010-glitch.png