The double fisherman's is one tough knot, it's true. But so is the constrictor (as in boa). Also called a gunner's knot, whip knot and timber knot, this particular knot is used to bind a rope very tightly to an object, such as tying a boat to a pole. If you'd like to tie a constrictor over something soft, the knot will hold better if you use a hard, stiff cord. If you intend to tie it over a hard surface, in contrast, a soft, stretchy one is best [source: Knots 3D].
Like the double fisherman's, the constrictor is a really difficult knot to untie. In some cases, it may be impossible, especially if you used a small line. In that case, the knot's binding force is concentrated over a smaller area, which gives it an extra hard bite. Sometimes, the knot can damage or scar the item it's tied around. If the constrictor doesn't sound ominous enough, there's also a double constrictor -- basically the same knot tied twice. Yes, it's even harder to untie [source: Knots 3D].
Another knot that can be difficult to loosen is the figure 8 loop. This knot is often used in climbing to secure the climber to the rope through his harness. If the climber falls, it's the figure 8 loop that will stop him from smashing into the ground (assuming it's tied properly). So it's a good thing it is hard to untie [source: Climbing Knots].
Are there any knots that are easy to undo? Certainly the bowknot, the one you tie your shoelaces with, is. But there's a knot that's made specifically to be easily loosened, and that's the highwayman's hitch. This simple, quick-release knot is used when tying up a horse, for example. The knot was supposedly given its name centuries back, when bandits — then known as highwaymen — tied up their horses with these knots when they ran off to quickly loot a spot [source: Action Donation Services].