Seems to be about the mileage many of these 5.7L Hemi engines grenade, due to the valve seats coming loose from the heads... That would be a design flaw, not an initial assembly failure.
I would assume by "assembly failure" they meant the assembly (rotating?) failed, not a failure due to incorrect initial assembly...
I'm thinking that a valve seat is what failed, and they're just telling the OP a basic reason to "not get too technical" ... the receipt should have the tech's findings in his own words, although they may have been edited by whoever printed the final receipt.
My car had performance heads and still dropped a valve seat with only about 20-25k miles on those particular heads, about 45k miles on the engine.
Needed a new engine.
About $4k later, after another $4k in earlier repairs from a broken valve spring, the car got traded in since I lost faith in its reliability ... I miss the AWD, but my FWD Focus has been awesome so far in the snow, and the Mustang is a lot more fun than the Charger ... completely different animal of course, but still ...
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Mine: '13 Mustang GT, Sterling Gray, 6-spd man, Recaros, Track Pk, 2.3L Whipple, 608 RWHP, 517 RWTQ
Fiancee's: '08 Mustang GT500, White/blue stripes, '13 TVS & injectors, FRPP twin-65mm TB, JLT 123mm CAI, 643 RWHP, 608 RWTQ
Previous: '07 Charger R/T AWD, heads/cam, a few other things. 355 AWHP, 370 AWTQ