Gap between inner and outer walls only on one surface.
I am trying to print a cookie cutter that I have been printing the past two years, recently whenever I go to print it there is a gap between the inner and outer walls but only on one side and a couple other patches. I have checked my Z offset, tensioned by belts, switched filaments and got my bed leveled with the bed level correction , no difference. What could be causing this? Im using the same file that I have been using the past two years and have printed thousands of these, they all came out perfect until this recent issue. 
RE: Gap between inner and outer walls only on one surface.
Probably the issue is the the uneven extrusion due to bowden ptfe tube between the extruder and the printer head being bent depending on the x position. Check for the wear of the tube - there are some forum threads related to this.
Sometimes rotating the model 90 degrees helps.
See my GitHub and printables.com for some 3d stuff that you may like.