I guess I should be clear that by "irreversible" I mean a transformation like the following: "to cut the yarn with scissors, to untie a knot that was strongly bound, or to felt together." So a slip knot is "reversible" in the sense that if you tug on it, it easily comes undone, whereas an overhand knot would just get tighter. You can think of felting as being equivalent to tying a lot of overhand knots between adjacent strands, they become permanently attached and could only be torn from each other.