colored horizontal lines across screen when shutting down imac

By any chance do the colorful lines you describe resemble this?

Your posted photo - I am getting exactly that on boot up after a clean install of Sierra 10.12.3 on my Mid 2011 iMac. It happens momentarily. I did not have the issue prior to the install. I appreciate any info you can give.

It may be a hardware problem that only Apple can fix, or it may be something they will eventually address with an incremental macOS update. Either way, given the fact your Mac is nearly new you should contact them: Contact Support.

I found a couple threads about this same thing, and I'm glad I did, because it seems to be an issue with more than just you and me. I too get the color horizontal lines down the whole screen on shutdown, and even then, only sometimes. I called Apple, and the technician I spoke with said he would run it by engineering to find out if it's "expected" behavior with the given graphics card installed. I can't believe that this behavior would be expected, but I gave him the benefit of the doubt. I never heard back. But since finding threads here about other people having the same problem, I must say I feel better about my problem...Lol.

Someone said in another thread that there was a recall on a specific graphics card some time ago, but it's not the card I have.

I have the same problem with a late 2015 27 inch iMac -- occasional horizontal colored lines filling entire screen on shutdown. In addition, I have another problem that happens on startup every time. As loading bar moves from left to right, it pauses just below the apple symbol and there is a very brief and small set of black and white horizontal bars to the left of the symbol. Then loading bar continues to right and machine boots up.

Anybody have this issue?

To test if this issue is related to Sierra 10.12.3 I reinstalled El Capitan via a clean installation and the issue is gone. I rebooted several times and the horizontal bar does not appear.