What is an exception to the rule that liquids are less dense than solids.

Water! Water is more denser than ice, which is H2O's solid form.

As far as I know the density is a property of material independent of quantity or shape. Liquids tend to be less dense than solids of the same material. But water is an exception to this rule. The density of solid water, or ice, is less than the density of liquid water.