Hi! You don't need to use "subassembly" functionality to reproduce this bug : you only have to use the symmetry mode on a group of parts. 1/ Put any part in the middle (let's say, Mk2 Inline Cockpit ) 2/ Attach another part (hmmm Small Hardpoint ?) to one side of the Cockpit 3/ Attach a part to the Small Hardpoint (we aren't using any symmetry mode yet), a fuel tank for example... 4/ Switch to mirror symmetry mode, take the Small Hardpoint and attach it again... and we have nod a symetric "plane" 5/ Now, try to attach in mirror symmetry mode to the fuel tank... hmmm.... a fuel line? (good luck ) I've tested with a rocket fuel tank, a jet fuel tank, some structural parts... same result : can't attach anything to them using mirror symmetry mode once i've "duplicated" them this way. When you select a root part of a group of parts and use mirror symmetry mode, all the child parts doesn't switch themselves recursively to mirror symmetry mode (and still use their default symmetry mode... radial?). Yep, sound like a bug (or it's the strangest feature ever ><) Workaround: If you have this bug, re-attach each part of the group in mirror symmetry mode, root part of the group first, then his children, ect... ("recursively", as the game should do) You should avoid to build your plane by attaching the parts on one side only and using the mirror symmetry mode to duplicate them (until the bug is fixed ).