Our 70-ton Tranes here have kicked out on 'freeze warning' before; there's a strainer in the water loop at the evaporator that can clog, restricting flow enough to allow freezing to occur if the chiller is actively cooling.  It's so strange to have an overheating data center in subzero (F) temps.  The flow sensor in the water loop can sometimes get too cold and not register the flow as well.