Major graphic glitches on 27" Intel iMac

No, never. Try and boot the Mac in safe mode to see if the issues persist.

If they don't persist in safe mode, they're caused by en external piece of software, you might want to evaluate the software installed to your mac

If they are persist in safe mode, I am thinking of a hardware issue with your graphics card

Start up your Intel-based Mac in safe mode

1. Turn on or restart your Mac, then immediately press and hold the Shift key until you see the login window.
2. Log in to your Mac.
3. You might be asked to log in again.
4. On either the first or second login window, you should see “Safe Boot” in the menu bar.

If the issues persist, please Contact - Official Apple Support to set up hardware service for your Mac

Remember—your iMac does not have a video "card."

Starting with the 2012 models, all iMacs have their video hardware integral with and soldered to the logic board. If the GPU has failed, you need a new logic board.

There is little diagnosis you can do at home but:

1) Heat-related? If the image is fine for a short time right after startup and then starts to break up, you could have an iMac starved for cooling air. Please see my comments on iMac cooling issues and their mitigation in this recent thread:

iMac Maintenance - Apple Community

2) Does the screen image changes when you move the display on its hinge? That can indicate a bad display cable.

3) External monitor test. If you have or can borrow an external monitor, connect it and set it to mirror. If the image is normal on the external, the fault is likely in the display or its cabling.

If the screen errors duplicate on the external, the fault is most like in the video systems on the logoc board.

Of these, only #1 is an at-home fix. #s 2 and 3 need Apple's evaluation and a repair quote.

This is what a Safe boot does:

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

If your external monitor requires 3rd party software it wouldn't work. When you rebooted normally how did things go?

This is indeed a graphic cards failure.. Please Contact - Official Apple Support to set up hardware service

Graphics card failure in progress is my guess.

Thanks for the follow-up.

AJ

Let's hope the Safe boot cleaned out the cache files that were causing the problem and that it doesn't come back again. 🤞