A budget often comes with conditions or exceptions. “‘Exceptions prove the rule’—and wreck the family budget” was written by newspaper columnist Olin Miller in May 1963. “Family” is often left out of the quotation that is frequently dubbed “Miller’s Law.” Lawrence J. Peter wrote in Peter’s People (1979):
 
“MILLER’S LAW: Exceptions prove the rule — and wreck the budget. — Olin Miller”
 
“Miller’s Law” has been included in Arthur Bloch’s “Murphy’s Law” books.
 
   
Wiktionary: exception that proves the rule
Etymology
It is derived from the medieval Latin legal principle: exceptio probat regulam in casibus non exceptis (the exception confirms the rule in the cases not excepted).
Noun
exception that proves the rule
1. A form of argument in which the existence of a counterexample to a rule is used to demonstrate the fact that a rule exists.
2. (idiomatic) The rare occurrence of a counterexample to a rule, used to underscore that the rule exists
 
15 May 1963, Seguin (TX) Gazette, “Grist and Grits” by Olin Miller, sec. 2, pg. 2, col. 3:
“Exceptions prove the rule”—and wreck the family budget.
 
14 April 1964, Bridgeport (CT) Post, “Paragraphically Speaking,” pg. 20, col. 2:
“Exceptions prove the rule”—and wreck the family budget.
   
Google Books
Peter’s People
By Laurence J. Peter
New York, NY: Tower
1979
Pg. 203:
MILLER’S LAW: Exceptions prove the rule — and wreck the budget. — Olin Miller
 
Google Books
The Complete Murphy’s Law:
A Definitive Collection
By Arthur Bloch
Los Angeles, CA: Price Stern Sloan
1991
Pg. 73:
MILLER’S LAW:
Exceptions prove the rule—and wreck the budget.
 
Google Books
20,000 Quips & Quotes
By Evan Esar
New York, NY: Barnes & Noble Books
1995, ©1968
Pg. 279:
EXCEPTION
Exceptions prove the rule — and wreck the family budget. -Olin Miller
 
Google Books
The Greatest Joke Book Ever
By Mel Greene
New York, NY: Avon Books
1999
Pg. 209:
Exceptions prove the rule . . . and wreck the budget.