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When you have parasitic resistances and inductances on the ground return path, any current injection into the ground, or sudden changes in current injection will develop "voltage" on the return line with respect to the real ground.
Another way to think about it is to consider the device physics. Consider an nmos transistor pulling a node to ground (Vss). The nmos transistor sits inside a P-Well biased at Vss (0V). The threshold and other device parametrics are referenced to the potential of this Well. However the contact of this well to Vss is resistive so it can instantaneously be different from Vss +ve or -ve. If enough electrons are injected into the well in an instant of time then the well will become -ve with respect to zero volts and the device physics will change accordingly.
If not handled properly or accounted for at design, this can have catastrophic results.