I wrote a long reply about why this could potentially happen, but I'm not sure why it isn't there, perhaps I forgot to post before closing the window...
You can reduce chance of this happening by keeping the anchor in the same position, and making sure your camera is snapped to a pixel if not. Its much harder to do the latter with a perspective camera, so you could potentially also do it with a shader (eg. use the unity2d sprite shader) with pixel snapping.