The shoemaker always wears the worst shoes proverb
The noun "shoemaker" can be replaced by "cobbler".
Experts often fail to apply their expertise to benefit themselves or their families.
The phrase was first recorded in John Heywood's 1546 glossary A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue:
"But who is wurs shod, than the shoemakers wyfe,
With shops full of newe shapen shoes all hir lyfe?"
Another similar phrase was found in Robert Burton's The Anatomy of Melancholy, first published in 1621:
"Him that makes shoes go barefoot himself".
To tell someone the truth about something that they had not understood or received false information
I have to set my wife straight about what happened yesterday.