As Gardyloo says, it appears to be rarely enforced. The rule is that you must complete the RTW, as per the rules (generally back to the country of origin, but some exceptions). If you fail to complete the RTW, the legal exposure is simple: the airline is allowed to reprice the segments that you have traveled, using point to point fares, and this would come to some multiple of your original fare. The airlne can charge it back to the travel agent, though I have never heard of this occuring (but have heard of it happening for other rule violations). I have had airlines charge addcol to my credit card when ticket was purchased from them and a mistake was made in computing the taxes, and this was done a week later as an unrelated charge without contacting me first; most credit card companies will not automatically reverse such charges, you would have to prove that it is fraudulent.