This isn't octane related as I first suspected.
This is a physical impact of some sort. I need to check compression next. Probably not until tomorrow. I'm thinking broken valve. Drove 5-10 miles on it after I noticed the miss. I WAS on 4 cylinders, as with the waste-spark method, the other plug on this coil wouldn't have fired with a dead short on this one.
It MAY only be a piece of a valve. Just no way to know. I'll do compression first, then maybe take an initial look in there with a borescope through the plug boss.