So, discovered this little piece on accident.
It's 9am and my 14th hour in factorio straight, so less words, more gif.
http://gph.is/2gvtapM

Yeah, that's an unfortunate interaction with the water-fix mod. If you didn't have the water-fix mod installed, you could only place tiles on the buildable area. Just be careful with factory overlay controllers once you research them, removing a tile from under them destroys them permanently.

Ah, got it. I was never planning on using flooring in there anyways. Just misclicked and got surprised.