From the paper http://www.sciencedirect.com/science/article/pii/S0305054809003360 :
A system like LaTeX will not check these equations for correctness. Obviously also human readers have troubles spotting these errors (otherwise this would not have slipped through the review process). However if the authors would have run this model through a modeling language like AMPL or GAMS, this error would yield syntax errors.
Of course such a mistake can still happen when translating AMPL or GAMS to LaTeX by hand. Now and then I see requests for some tool to convert AMPL/GAMS equations to LaTeX automatically.