The cobbler's children have no shoes
Literally 'in the blacksmith's house, a wooden knife'. Describes the irony of experts or professionals who lack the very thing they provide to others.
The dentist has cavities. The cobbler's children have no shoes.
—The IT guy doesn't have a computer at home? —The cobbler's children have no shoes.
The chef eats terribly. The cobbler's children have no shoes.