Overfilling the keg so that the gas dip tube is below the liquid level, followed by higher pressure in the keg than what the regulator is pushing. Also, relieving the pressure on a warm keg full of beer can cause foaming which blows back into the gas line.
Check valves on the manifold will prevent this.
I don't think I have any. I would not get this if I had any right. Its a 3 way manifold
I would bet to say you do not have any. I have force carbonated a full kegs by shaking the heck out of them and never had beer in my gas lines.