Calc's current behavior when clicking "Sum" button on formula toolbar with x*y 
range selection does not seem to make much sense.
What Calc currently does is filling of selected range with randomly generated 
formulas (see repro steps below). I doubt that this is what user could ever 
want to happen, let alone want often. I suggest mimicking competing 
spreadsheet by inserting "Sum" below (and enhance behavior by inserting it 
above selected range or sideways, depending on if Ctrl and/or Shift keys are 
pressed).
Repro steps:
1. Open attached "summing-example.ods" in Calc.
2. Select B2:D11 range and click "Sum" button on "Formula" toolbar. Note how 
contents in all cells in the range is replaced by seemingly random formulas 
like "=SUM(B#REF!:B1)".

Suggested behavior:
2. Select range B2:D11, click "Sum". Calc should generate "Sum(B1:B11)", "Sum
(C1:C11)", "Sum(D1:D11)" in cells B12, C12, D12 respectively.

Suggested enhancement:
2. Select range B2:D11, hold Ctrl key and click "Sum". Calc should 
generate "Sum(B1:B11)", "Sum(C1:C11)", "Sum(D1:D11)" in cells B1, C1, D1 
respectively.
Holding Shift key should cause Calc to insert "Sum(Bx:Dx)" in cell Ex for 
every row of selection.
Combining Ctrl and Shift should cause Calc to insert sums to the left of the 
range.