So I built myself a subassembly of a camera with an aiming circle (hollow fuselage ( Frontscale and backscale 0.1,0.1 ), and when I put in the exact center of my aircraft, it will still get mirrored, thus causing the part count to go up exponentially after each mirroring!
Recreation
Yeah that can get annoying
I usually will just mirror parts individually when I need to instead of mirroring the entire aircraft