So, After a lot of digging (and me cleaning my room) I found the problem. My computer had a hiccup with the drive that steam was installed on right before this happened. This hiccup made steam forget that I had opted into the .15, so as far as I can figure, steam tried to set my file to .14, even ...

Game was working fine, randomly it's started crashing on start up during Building Prototypes...
I get the error "Error in assignID, item-sugroup with name 'fill-barrel' does not exist."
I'm not running any mods

Current file as follows
0.001 2017-05-02 14:09:39; Factorio 0.14.23 (build 25374, win64 ...