If it's 30 degrees or steeper or underneath terrain that is 30 degrees or steeper, it has the potential to slide. The "it's never slid before" thought process is a dangerous one. If it has snow on it and it's over 30 degrees, it can slide and Murphy's Law says that the first time you see it slide it'll be when your ass is on it.
"People say that marijuana smoking is going to get in the way of my career. I say to them that on the contrary, my fighting career is getting in the way of my marijuana smoking." -Nick Diaz