I disagree strongly with:
# 2 and # 9
"The second worst mistake, the survey found, was Andrew Johnson's decision just after the Civil War to side with Southern whites and oppose improvements in justice for Southern blacks beyond abolishing slavery."
Reconstruction was a disaster. After Johnson left office, Grant hammered the South creating deep resentments among Southern Whites. I don't understand why this mistake was listed since Grant reversed Johnson's moderate approach (except in Tennessee). Reconstruction led to the rise of the KKK and Jim Crow. Had more moderate policies prevailed, Civil Rights for Blacks might have evolved more naturally and sooner.
As for #9, Iran-Contra was not a good think, that is trading with Iran, but helping the freedom fighters in Central America was a good thing.
I see they left out FDR's caving into Stalin at Yalta; loosing China and Jimmy Carter abandoning the Shah in loosing Iran.
I rank Andy Jackson's threat in 1833 to put 50,000 Federal troops in S Carolina as a major error.
Occupations are rarely pretty.
ML/NJ
I suggest that the Reconstruction wasn't tough enough. Grant did not prosecute the traitors such as Lee, Davis, Floyd.
I submit one of the really bad blunders was ending the reconstruction before democracy was well established. The reconstruction was far LESS corrupt than the antebellum southern government, which skimmed money from projects to build fortifications, and gave the money to slave owners as rent for their slave's labor.