The Thief knot resembles the Reef knot at a casual glance. Note that the ends of the Thief Knot come off opposite sides of the knot. In the Reef knot, they come off the same sides.
However, the Thief knot has no strength whatsoever, and will slip under tension.
Try it and see!

Tip. Only use this knot for tricks. NEVER use it where life and limb are at risk.