As previously mentioned for a record to be set at a shorter distance the technical rules must be met for a world record:
1. The split location must have been measured by an A or B Level IAAF Measurer. The measurement must be confirmed by another Level A measurer. Plus the split location must meet the requirements for drop and separation. There are several major marathons that the overall distance meets these specs but not some of their splits.
Interestingly, the split does not need to be Certified for a WR!
2. Timing must meet all specification. Another interesting point is that the official times are rounded to the next full second which means if two runners cross the 30k point in record time and one is a few meters ahead of the other - there is the possibility that both runners could have the same official time and therefore share the world record!* Some of you might scratch your heads over this but time (or distance in field events) is different then placing. If two sprinters both break the WR in the 100m but one edges out the other with the same official time, then they both share the WR
3. The runner(s) also must finish the entire advertised distance of the race. Most race director don't realize that they can get around this by advertising the race as the XYZ Marathon with the Invitational 30k. In that case if a runner did set a record at the 30k and didn't finish, then he would get the WR.
4. The runner must be drug tested. There have been situation where they only tested the top 3 males and females at the finish and neglected to test the athlete who set the split record.
At one recent world class marathon, it was the two pacesetters who crossed the split point at a WR record time. They both stepped off the road shortly after. The next runner who crossed the point finished in the top 3 so he got credit for the record, but the next guy who was just behind him with the same time was not in the top 3 and got screwed out of the WR.