When I was six, I got the Vladek’s Dark Fortress set for Christmas, and it didn’t have the saddle piece for Jayko’s horse. Therefore, I was never able to properly use the horse-mounted catapult. 
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In 2012, I got the Fire Brigade modular building for my birthday. After opening it up, I noticed that one of the firefighter mini figure torsos was missing. In its place was a police officer’s torso 
I had spare uniforms, so I didn’t have to order a replacement, but to this day, a police uniform is sitting on the back of my Fire Brigade. I think my brother put a spare city head on it at one point
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A few times, here’s some examples of the top of my head.
Back when X-Pods were a thing, I had this Arachnid pod but it was missing a piece. Can’t remember exactly which one, but when I contacted LEGO, they gave me not just the replacement part, but another WHOLE COPY of the set!
I’ve bought a Harry Potter Advent Calendar after holiday 2019, from Walmart at clearance price, but only 3 minifigures were missing: Harry, Ron and Hermione. Coincidentally they were all on the same row of the box towards one side, and yet the box was still sealed, not battered open. I contacted LEGO and they sent all the parts to make those 3 figures.
Maniac4Bricks (Joey)
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