If you ever are missing a piece in a Lego set, they send you them for free in these tiny Lego packages. : r/mildlyinteresting
My treehouse set mist a piece. First time to have a missing piece in a new set. luckily Lego support helped me out! Love their fast service. : r/lego
LEGO Flat White Parts - Various Sizes - Spare Missing Parts | eBay
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I've had the very rare missing piece before, but have never received a piece that (according to the parts list) isn't even in the set! Bag 2 in the new Space Shuttle