Unity-gain closed loop Miller OPAMP fails to follow the input after a few microseconds?
If the simulation suddenly changes state like this, it is probably due to numerical noise, but the design has a "resonance point". Numerical noise is really small, but it can still affect the simulation if the design has issues. 

You would probably see this kind of thing in reality, if you could physically design the circuit with zero capacitance (which is impossible). A pcb naturally acts as a low pass filter for high frequency noise. 

The numerical noise can be reduced by increasing cshunt (a small capacitor tied to every node, adding a capacitor to the output (thus creating a low pass filter and reducing the bandwidth of the circuit and noise).

It is really useful to understand real world parasitics and know when and where to use them in your simulation. Large resistors (&lt;100M) can be added to simulate air, and increase numerical stability. Capacitors on the order of pF's can be added from signal to ground to simulate the capacitance of a PCB. Inductors can be added to simulate wire or pcb traces.