That's a rough one. Mine was fixing some physics/flight aerodynamics software that had been generated by a Fortran-to-C compiler (e.g., http://www.netlib.org/f2

That's a rough one. Mine was fixing some physics/flight aerodynamics software that had been generated by a Fortran-to-C compiler (e.g., http://www.netlib.org/f2c/ http://www.netlib.org/f2c/). It was in inscrutable mess and couple-years-out-of-academic-physics-turned-SE me thought I'd be able to do it easily since I had so much Fortran and C experience. I failed quite hard at that.