Usually if you have a problem where the circuit works in the simulation but not in real life, the problem is not with the accuracy of the simulator, but with something you left out of the simulation. For example, in your simulation you usually assume all circuit elements are connected directly to the power supply, but in real life the elements are connected to the power supply through wires which have resistance and inductance. Or perhaps you assume that all elements are at the same temperature when in real life power dissipation in the elements makes them have slightly different temperatures. A very common problem is in statistical variation of the elements. I don't know if you are talking a circuit made with discrete elements or an integrated circuit, but in both cases the elements vary. How robust is your circuit if the elements don't have the values (width, length, resistance, capacitance, etc.) that you assumed, but instead vary by say 20%? What if one is 20% too large and another is 20% too small? Try thinking along these lines and see if you can make your circuit robust enough to tolerate variations.