How do they make a bug like this?

For a small project I needed to solve a very small QP model. As the input and output were in Excel, it made some sense to try Excel’s Solver. One of the constraints is:

The way to do this is to create a cell with the sum and then use Solver.SolverAdd to add a constraint. The complete call can look like:

Call Solver.SolverAdd(CellRef:=sumcell, Relation:=2, FormulaText:=1)

See http://msdn.microsoft.com/en-us/library/office/ff838657(v=office.14).aspx

Unfortunately (for me) this did not work and I see:

After much experimentation I used instead:

Call Solver.SolverAdd(CellRef:=sumcell, Relation:=2, FormulaText:=0.999999999999999)

Now I see:

How in the world can such a bug be introduced? Some bugs make sense, but the logic of this one is difficult to fathom. I cannot but think there must be a statement somewhere:

if rhs=1 and (not interactive) and (user=’Erwin.Kalvelagen’) then
    call delete_constraint_to_confuse_user

Solver works correctly with a =1 equation when used from the GUI.
Well, found some more reports on this strange bug:

• http://www.pcreview.co.uk/forums/problem-solveradd-can-anyone-help-t961905.html
• http://excelforum.com/excel-programming-vba-macros/492647-solved-missing-condition-after-solver-called-by-macro.html
• http://stackoverflow.com/questions/15620177/excel-solver-ignoring-constraint-in-vba

This bug is present in Excel 2010 and 2013 (32 and 64 bit).