Program: 65790
Project: 65790
Buggy source:
public void selectionChanged(SelectionChangedEvent event){
  String sheetName=(String)((IStructuredSelection)event.getSelection()).getFirstElement();
  String queryText=this.getInitializationDesign().getQueryText();
  if (sheetName.equalsIgnoreCase(currentSheetName) && isNewFile(queryText,getFileName(selectedFile))) {
    flag=false;
    return;
  }
 else   if (flag) {
    flag=false;
    return;
  }
  if (currentSheetName != null && !MessageDialog.openConfirm(worksheetsCombo.getControl().getShell(),Messages.getString("String_Node_Str"),Messages.getString("String_Node_Str"))) {
    flag=true;
    worksheetsCombo.setSelection(new StructuredSelection(currentSheetName));
    return;
  }
  currentSheetName=sheetName;
  setPageComplete(false);
  selectedColumnsViewer.getTable().removeAll();
  savedSelectedColumnsInfoList.clear();
  availableList.removeAll();
  if (this.getShell() != null) {
    Cursor waitCursor=new Cursor(this.getShell().getDisplay(),SWT.CURSOR_WAIT);
    this.getControl().setCursor(waitCursor);
  }
  String fileName=getFileName(selectedFile);
  String[] columnNames=getFileColumnNames(selectedFile);
  if (columnNames != null && columnNames.length != 0) {
    enableListAndViewer();
    availableList.setItems(columnNames);
    availableList.select(0);
    btnAdd.setEnabled(true);
    btnRemove.setEnabled(false);
    btnMoveUp.setEnabled(false);
    btnMoveDown.setEnabled(false);
    if (!(fileName.endsWith(ExcelODAConstants.XLS_FORMAT) || fileName.endsWith(ExcelODAConstants.XLSX_FORMAT))) {
      setMessage(Messages.getString("String_Node_Str"),WARNING);
    }
 else {
      if (selectedColumnsViewer.getTable().getItemCount() == 0) {
        String errMsg=currentSheetName == null ? Messages.getString("String_Node_Str") : Messages.getString("String_Node_Str");
        setMessage(errMsg,ERROR);
      }
 else {
        setMessage(DEFAULT_MESSAGE);
      }
    }
  }
  if (this.getShell() != null) {
    Cursor normalCursor=new Cursor(this.getShell().getDisplay(),SWT.CURSOR_ARROW);
    this.getControl().setCursor(normalCursor);
  }
}

Fixed version (ground truth):
public void selectionChanged(SelectionChangedEvent event){
  String sheetName=(String)((IStructuredSelection)event.getSelection()).getFirstElement();
  String queryText=this.getInitializationDesign().getQueryText();
  if (sheetName.equalsIgnoreCase(currentSheetName) && isNewFile(queryText,getFileName(selectedFile))) {
    flag=false;
    return;
  }
 else   if (flag) {
    flag=false;
    return;
  }
  if (currentSheetName != null && !MessageDialog.openConfirm(worksheetsCombo.getControl().getShell(),Messages.getString("String_Node_Str"),Messages.getString("String_Node_Str"))) {
    flag=true;
    worksheetsCombo.setSelection(new StructuredSelection(currentSheetName));
    return;
  }
  currentSheetName=sheetName;
  setPageComplete(false);
  selectedColumnsViewer.getTable().removeAll();
  savedSelectedColumnsInfoList.clear();
  availableList.removeAll();
  if (this.getShell() != null) {
    Cursor waitCursor=new Cursor(this.getShell().getDisplay(),SWT.CURSOR_WAIT);
    this.getControl().setCursor(waitCursor);
  }
  String fileName=getFileName(selectedFile);
  String[] columnNames=getFileColumnNames(selectedFile);
  if (columnNames != null && columnNames.length != 0) {
    enableListAndViewer();
    availableList.setItems(columnNames);
    availableList.select(0);
    btnAdd.setEnabled(true);
    btnRemove.setEnabled(false);
    btnMoveUp.setEnabled(false);
    btnMoveDown.setEnabled(false);
    if (!(fileName.endsWith(ExcelODAConstants.XLS_FORMAT) || fileName.endsWith(ExcelODAConstants.XLSX_FORMAT))) {
      setMessage(Messages.getString("String_Node_Str"),WARNING);
    }
 else {
      if (selectedColumnsViewer.getTable().getItemCount() == 0) {
        setMessage(getEmptyColumnErrMsg(),ERROR);
      }
 else {
        setMessage(DEFAULT_MESSAGE);
      }
    }
  }
  if (this.getShell() != null) {
    Cursor normalCursor=new Cursor(this.getShell().getDisplay(),SWT.CURSOR_ARROW);
    this.getControl().setCursor(normalCursor);
  }
}