In which cases prior to the Greek crisis did the IMF admit mistakes in it's economic policies? Has the IMF ever admitted mistakes on it's economic policies prior to the Greek crisis?
Yes.


  The International Monetary Fund yesterday admitted that its mistakes
  helped plunge Argentina deeper into the red during the currency crisis
  that crippled the country's economy three years ago.
  
  In a report published yesterday by its independent evaluation office,
  the IMF said it ought to have prevented the Argentine government from
  following poor economic policies.
  
  "IMF surveillance failed to highlight the growing vulnerabilities in
  the authorities' choice of policies and the IMF erred by supporting
  inadequate policies too long," it said.


IMF admits mistakes in Argentina crisis By Edmund Conway

The actual report is here: IMF's Independent Evaluation Office Announces Release of Report on the Role of the IMF in Argentina