Consider this circuit with a sinusoidal source connected to the collector and a pulse toggling between 0 and 5 connected to the base of a 2N2222 transistor.
What I imagined the output being was when the input voltage is 5 the transistor turns on and we would see the sine wave at the emitter too. And when it's 0 it's off and we get 0V at the output. Exactly like a switch. Right?
But after I simulated it in PSpice, I'll get this. When the input voltage is zero and the base is basically grounded, you still get the negative part of the sine signal. (Red=Vout, Green=Vin)