Very tricky - I believe it is still technically possible for this to be done. Essentially, all the segments would need to be booked in another PNR, and then perhaps you need to change date or something, so all segments are deleted. After a while, the original PNR gets purged, and only the ticket is available. A new PNR is then created, and certain manutal entries are used to link the original e-ticket with the segments in the new PNR.

Another possibility is that when different systems are involved. For example, an agent using Sabre (either a TA or an AAgent) created the PNR and issued the RTW. The original ticket contains LA segments, which is on a different partition of Sabre (so a different PNR). Then, you asked another airline to do date change or re-issue, so the LA segments get further booked into a different PNR because other airlines using Amadeus would create a new LA PNR when booking LA flights.

Historically, most airlines use their own systems, so you have many PNRs and a lot of cross-platform issues (or loopholes). Now, all oneworld airlines except AA and LA use Amadeus, so these kinds of things are less likely to occur.

Back to the question, if someone were to attempt to create two PNRs with segments of the same airline that should be married, I suppose the agent would be looking for an ADM. Sure, the agent may also get lucky and not get caught.