In LTspice IV, I am simulating a 1N914 diode behavior in response to a variable current source in two circuits. One circuit has just the current source and the diode, and the other has the current source and a the diode connected in parallel with a 1 kilo Ohms resistor. I vary the current out of the current source from 0 mA to 2 mA, plotting the forward voltage across the diode on the left and the current through it on the right, both on the vertical axis, versus the current out of the current source on the horizontal axis.
As shown in the picture, when I put the horizontal cursor line at the voltage where the forward voltage (V(D1)) is almost 540 mV, different values of current through the diode appear in each circuit ( I(D1) ). Shockley equation tells that if the forward voltage is constant, then the forward current must still the same.