Anything that prevents you from playing the game properly. Do you have issues playing for the game, downloading it or successfully running it on your computer? Let us know here.
So I up myself to the latest experimental build (0.13.13 I think?) and saw that we could download a few mods. After I was done going through them, it restarted the game and now the game won't load, saying that it's missing files for the Resource Spawn Overhaul mod. I looked around and saw that I could delete mods from my user/roaming/factorio but when I went into Roaming folder, there's nothing there.
To get into the correct "Roaming" folder, try to type :
Start > Run > %APPDATA%
And hit Enter
There, you should find a Factorio folder, and a mods folder in it.
Koub - Please consider English is not my native language.