How do I avoid the black bars after opening to replace HDD?

I’ve decided to upgrade my iMac 2017 internal fusion drive, but I’ve seem to many folks who tried the same thing and ended up with a screen with black bars on the right.

All these people who ended with a screwed up monitor were careful and used the iFixit cutter.

Does anyone have any idea of what went wrong and how can I avoid the same mistake?

Here’s a photo of what many folks are experiencing.