I think Remick’s gory death in TNG’s Conspiracy comes pretty close. I’m not even going to link to an image, it’s that gruesome.

The most shocking and gruesome death in all of TOS wasn’t bloody or particularly violent.

This scene freaked me out when I was a kid, and I still don’t like watching this part. It also played against viewer expectations that the redshirt always bites it.
On the flip side of the same coin, I wonder if there were bloopers where the rest of the cubed crew got accidentally stepped on?