The device model is typically expected to be accurate under normal working conditions. Error states (those that do not occur under normal conditions) should never be entered. Most errors are hardware induced; e.g., parity errors, failure to read firmware ROM data, etc. These will never occur in Simics, because the virtual hardware is controlled by the simulator. The device model is simplified by not having to represent the error states.
Sometimes, though, the errors are the interesting parts of the model. If the model is to be used in developing the error handling in a device driver, the error states need to be modeled in more detail (and some means of triggering the errors must be added). Fault-injection in simulated networks is another example.