Think about it for a moment, tho, and you'll soon see how impractical it is--unless the assumption is that anyone building a LEGO kit will first lay out all the pieces and compare them to the list in the instruction manual. There might be people who would do that, but I'm not one.

So, contacting LEGO for replacement pieces would necessitate disassembling the model to add in the missing elements.

(A few years back I had the decidedly un-fun experience of having to almost completely disassemble a light table I'd put together when literally the next-to-last part would not fit properly. Re-examining the illustrated assembly instructions I discovered one of the earliest pieces was drawn backwards and the table would not go together until I took it apart and started over.)