Here's 2 more for you, both good stories to offset all the mat missing cheaters out there...
1) A case of "big negative split can be legit"...A guy on my XC team about 5 years ago ran splits of something like 1:55/1:35 at Philly. He was a ~29:30 8k guy so the 2nd half split is plausible. Why the huge discrepancy? First 10 miles he paced a friend at 10:00/mile pace, then he ran in at 7:00 pace. Shows you splits alone don't tell the whole story (95% of the time they do).
2) A case of "yes there are honest runners out there"...Checking the results for the race I work for in 2008, had a runner who missed the 20 mile chip mat and had 1st half/2nd half splits that were out of line. Before I even get into the office Monday morning I've got an email from this guy that says; "Pull me from your results. I stopped right after mile 19 and walked back to the finish area to turn in my chip and meet my family (mile 19 is right next to mile 23). When I got to the finish area my kids were excited to see me so I ran through the finish line with them." How cool that this guy was proactive and understood that race results mean something no matter where you finish.