File removal depends upon the position in the canal, canal curvature, access and patience (you need lots of this!). It is most difficult to remove files that have separated beyond the curvature and close to the apex.

This file tip separated just before the curvature.  It is most important not to perforate while attempting removal in those canals with sharp curvatures. 

Let’s be honest, if you haven’t separated a file in a canal you haven’t performed a root treatment.  It happens!  Now, the thing is with a bit of knowledge and skill (and luck!) you can remove the separated file tip beyond a sharp curvature without perforating the root.