Holy crap. It's called con crunch for a reason. An hour ago I was like, I am way too tired and frustrated for this, gotta be careful not to make a huge mistake like accidentally sewing the back of the coat to the front.
I did it. I sewed the back to the front.
Fortunately, just a couple cm, immediately opened it up again and for whatever reason it's barely visible (contrary to a couple other spots), but... fuck.