I see that "Mistakes were made" has its own Wikipedia page:
The New York Times has called the phrase a "classic Washington linguistic construct." Political consultant William Schneider suggested that this usage be referred to as the "past exonerative" tense, and commentator William Safire has defined the phrase as "[a] passive-evasive way of acknowledging error while distancing the speaker from responsibility for it." While perhaps most famous in politics, the phrase has also been used in business, sports, and entertainment.The phrase is most associated with Richard Nixon and his press secretary Ron Ziegler:
U.S. President Richard Nixon used the phrase several times in reference to wrongdoings by his own electoral organization and presidential administration.But it's not as though the association with Nixon has worked over the years to warn off other politicians. As you can see at the Wikipedia page, President Reagan used it in 1987 (about Iran-Contra, President Clinton used it in 1997 (about Democratic Party fundraising scandals. Senator McCain used it in 2005 (about the Iraq war). There are more items on the list, including — I love Wikipedia —today's statement by the IRS.
On May 1, 1973, White House Press Secretary Ron Ziegler stated "I would apologize to the Post, and I would apologize to Mr. Woodward and Mr. Bernstein" (referring to Bob Woodward and Carl Bernstein of The Washington Post). He continued, "We would all have to say that mistakes were made in terms of comments. I was overenthusiastic in my comments about the Post, particularly if you look at them in the context of developments that have taken place." The previous day, White House counsel John Dean and Nixon aides John Ehrlichman and H.R. Haldeman had resigned, as the Watergate scandal progressed.
Under the "see also" heading: