The LO side of a Mosfet Half Bridge Gate Drive
The way you have wired it up in that simulation, there is a 1kOhm load being driven by an 11.9V power supply when the MOSFET is off. In that circuit when you turn the MOSFET on, it would short out the 11.9V power supply to ground. This seems a tad odd? It would be helpful if you posted a schematic of the exact circuit you have wired up.

Anyway, what is connected to the Gate of the MOSFET on your breadboard? If it is floating, you may find that it switches itself on if there is charge on the gate. This is especially likely if you had the gate connected to a voltage source and then unplugged the wire. In this situation you should have a resistor (10k to 100k or so) connected between the gate and source to ensure that the transistor is turned off.