Catastrophic bicycle failures

[GirishH]

As some of you know, I bought a new bicycle that failed catastrophically within 3 days..

​The gears were slipping and skipping right from day zero. It’s a 3 x 8 setup, and the shop mechanic claimed the manufacturer had mistakenly installed a 7-speed rear derailleur instead of an 8-speed. Whether that's true or not, he eventually swapped it out for a new 8-speed derailleur after my complaints.

​After that, the bike rode fine for three days—maybe 150 to 200 km.
   

​Then, on day four while riding through flat terrain, the shifting went haywire: a single lever push would suddenly jump 3 to 4 gears at a time. Within 15 to 20 minutes, I heard something fly off the rear of the bike. I looked back and saw the chain had snapped and wrapped tightly around the cassette, while the rear derailleur had completely sheared off and was lying on the road.
   

   

​The mechanic insists I must have crashed the bike or hit it against something. That’s entirely untrue—there isn't a single scratch on the frame or anywhere else.

​I'm currently fighting the manufacturer over this, but I'm trying to figure out what technically caused such a catastrophic failure.