The line became famous and has been repeated frequently in New York City history.
12 February 1941, New York Times, pg. 9:
Mayor Admits O'Brien
Appointment a Mistake
Just before he left the stand, Senator Clark reminded Mayor La Guardia that he had appointed Judge O'Brien to the bench. The Mayor chuckled.
"Senator, I have made a lot of good appointments and I think I am good," he replied, "but when I make a mistake, it's a beaut."
30 July 1942, New York Times, pg. 23:
Mayor Gives His Support to Marcantonio;
Chided By Antonini for "Beaut of a Mistake"
"The Mayor once said that when he makes a mistake it's a beaut," said Mr. Antonini. "Well, this is a beaut, all right."
24 September 1942, Los Angeles Times, "Fair Enough" by Westbrook Pegler, pg. A?:
Mayor LaGuardia said, in criticism of some judge whom he had appointed and who was displeasing him, "when I make a mistake, I made a beaut," and the people of New York may say the same.
14 February 1960, Los Angeles Times, "Firello was quite a fellow" by Bennett Cerf, pg. 4:
By his own admission, when he made a mistake, "It was a beaut."
6 January 1972, Washington Post, pg. F1:
"When I make a mistake, it's a beaut," the late Fiorello LaGuardia used to say.
22 February 1987, New York Times, "On Language" by William Safire, pg. SM10:
"I'M NOT KNOWN TO make many mistakes," said Senator Lloyd Bentsen modestly, in extricating himself from a fundraising furor, "but when I do, it's a doozie."
This is a botched use of a statement by New York Mayor Fiorello H. La Guardia: When I make a mistake, it's a beaut." The Little Flower's confession became one of the great observations in political wisdom, comparable to Woodrow Wilson's suggestion that an opponent in difficulty should not be attacked, expressed as "Never murder a man who is committing suicide," and Navy Secretary Claude Swanson's paean to political loyalty in the New Deal era, "When the water reaches the upper deck, follow the rats."
21 October 1990, New York Times, pg. E18:
As he said of himself in regard to one of his judicial appointments, Mayor La Guardia would also have said of his move to Gracie Mansion, "When I make a mistake, it's a beaut."