I loaded ArcGIS 10.3 recently, a jump from 10.2.2.
In the process I discovered something interesting. We use many different projection files. In the past I have used a text editor that allows me to do some specific replace request.
For example the .PRJ files are one long string and it is hard to edit so I replaced "[" with a carriage return in front of it, in my case this was replace: [ with: ^P[
What this gave me was a file that went from
PROJCS["NAD_1983_Sheridan_Surface_SF-1.000235",GEOGCS["GCS_North_American_1983",DATUM . . . . .
to a file that looked like this:
I did this because it made finding the X, Y and Scale_Factor easier when editing them.
In the past ArcGIS has not had an issue with this format. I have discovered with ArcGIS 10.3 it will not recognize the .PRJ file if there are "Carriage Returns" in it, although most of my other programs still do. So I am going back through my stockpile of Projection files and removing the Carriage Returns now.
Interesting so I thought I would share.