Mosfet Threshold Voltage
Some FlipFlops use PassGates to switch between Feedback (store a bit) and Update (change the bit).

The PassGates look like this







simulate this circuit &ndash; Schematic created using CircuitLab

Notice the FETs are 4-terminal. As the signal varies, the gate-channel voltage varies, and the channel resistance varies, and the FF setup/hold times will vary.