I have VBA code that copies the first row and pastes the values to multiple rows. The below code runs fine and pastes the rows as expected:
Sub Macro1()
  Dim i As Long
  Application.Calculation = xlCalculationManual
  Range("A1:M1").Select
  Selection.Copy
  For i = 1 To 50
    Range("A" & i).Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
      :=False, Transpose:=False
  Next i
End Sub
However, if I move Application.Calculation = xlCalculationManual down two lines as below, then the code throws a 1004 run-time error:  
Sub Macro1()
  Dim i As Long    
  Range("A1:M1").Select
  Selection.Copy
  Application.Calculation = xlCalculationManual
  For i = 1 To 50
    Range("A" & i).Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
      :=False, Transpose:=False
  Next i
End Sub
I've searched for information on the VBA language reference site here: http://msdn.microsoft.com/en-us/library/office/jj692818(v=office.15).aspx and the Excel developer reference site here: http://msdn.microsoft.com/en-us/library/office/ff194068(v=office.15).aspx.
Further, I've verified this error using both Excel 2010 running on Windows 7 and 2013 running on Windows 8.1.
Can someone help me understand why the location of Application.Calculation = xlManualCalculation would affect how the code runs?
EDIT:
I ran some additional tests to check if focus is lost or the clipboard is cleared. First to see if focus is lost I recorded a Macro that copied the first row with ctrl + x, then I changed the calculation mode of the workbook, then I hit ctrl + x again without re-selecting the cells. This is the resultant Macro:
Sub MacroFocusTest()
    Range("A1:M1").Select
    Selection.Copy
    Application.CutCopyMode = False 'Macro recording entered this.
    Application.Calculation = xlManual
    Selection.Cut 'Range("A1:M1") is cut on the worksheet suggesting focus was not lost.
End Sub
Next, I entered a variable into my original Macro1 to capture the Application.CutCopyMode at various stages of execution. Follows is the resultant Macro:
Sub Macro1()
  Dim i As Long
  Dim bCCMode as Boolean    
  bCCMode = Application.CutCopyMode ' False
  Range("A1:M1").Select
  Selection.Copy
  bCCMode = Application.CutCopyMode ' True
  Application.EnableEvents = False ' Included because I mention in comments no error is thrown using this
  bCCMode = Application.CutCopyMode ' True
  Application.Calculation = xlCalculationManual
  bCCMode = Application.CutCopyMode ' False
  For i = 1 To 50
    Range("A" & i).Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
      :=False, Transpose:=False
  Next i
End Sub  
Based on the results of these two tests I believe that Application.Calculation = xlCalculationManual does not cause the range to lose focus, but does clear the clipboard.
 
     
     
    