I don't know about other states but in Illinios certain traps on drain lines are not allowed. The most common one that I see is the S Trap which is very common in older homes that have had a second or third bathroom installed in the past few years and the homeowner had done the work themselves. I have also noticed them in newer homes where a bathroom or sink was added to the basement or laundry room and the homeowner did the work themselves or had a friend do it. Every now and then I will hear a seller say that they had a licensed plumber do the work but that is rare. For those of you that are not familiar with S Traps or why they are a problem An S-trap is found in the drainline under the sink. S-traps have a tendency to siphon out the liquid seal in the trap when a basin of water is emptied.
Below are examples of different traps that are not allowed.
Jim Watzlawick, Watz Home Inspections, Illinois Home Inspector