Has anyone's screen fallen off their late 2012 iMac?

It's a feature. The new fanless design iMacs whereby the screen falls out to aid cooling during high CPU intensive activity.

The screens are stuck on and require heat to loosen off the bonding strips that hold the screen on. These are one-time use and are replaced should the system require dismantling.

Has it been dismantled or repaired in the past, by Apple or otherwise?

Can you feel the sticky bonding strip around the back of the screen or around the chassis itself? If it isn't sticky or not there, then that's the reason it fell out.

On the plus side, if you fancy a hard drive upgrade, now's your chance.

I can't speak for the iMac, but I repaired an iPhone a while ago, and the bonding on that was like super glue. It needed a lot of heat for a good ten minutes before it started to give. It could be that the company who did the upgrade didn't replace the bonding, but used the original (and compromised) one.

I would say that unless the company you dealt with for the upgrade are Apple approved, then you may have a problem. I think there is a strong chance that Apple wouldn't be interested and you may even have affacted your Applecare warranty.

I would suggest you speak to the company who did the upgrade first and see what they say. Can you see if the screen connector cable been damaged?

Good luck.

I agree, any claim now is against the repairer, photograph and document what happened in detail. Then approach the company (politely) and firmly tell them that you expect a full and proper repair.

Good luck, post back and keep us informed.