Originally Posted by mazinger_z
Holy crap! (no pun intended) So, it is possible?
So, does incoming water use those same pipes, too? I doubt it. Aren't they under pressure, and/or a distinct separate part of the plumbing system?
The incoming water and the drain system are two completely separate systems. They do not interconnect in any way.
What happened in your friend's case (apparently) is that the use of pressure to clear the blockage in another apartment caused sewage and other drain water to come out of the drain pipes in your friend's sink.
The water coming out of the faucets is still clean water (although it would be a good idea to thoroughly wash the faucets themselves).