I was using a cockpit block, size was changed to less than 1/6 of the normal one. Whenever I tried to mirror my plane, it mirrored the cockpit too, even though it was right in the middle of the plane. Having more than 500 of these is not fun removing.
use the mirror function in the rotate area
And don't mirror 500 times.
Don't mirror the entire plane. Do it part by part. It's longer but more safe.