A Square Knot will slip if not under tension and WILL slip when tied with Nylon rope.

Is a square knot secure?

The square knot, also known as the reef knot, is first and foremost a binding knot. Its primary function is to secure a line tightly up against an object as when tying a bandage, a package or the flaps of a wall tent at camp.

Will a square knot come undone?

Square knots can easily slip or come undone with movement or jam under a heavy load, so never use them for holding a critical load or supporting a person [source: Animated Knots]. ... Pull on the ends of the ropes to tighten, completing the knot. You can add extra half-knots (half-hitches) for security.