It's the study that compromised journalistic integrity, leading some journalists to agree not toobtain outside commenton the paper before an embargo lifted. It's the study that featured shocking images of tumors bursting out all over hapless rats, images reproduced in various stories online in all of their tumoristic, gross morphological horror. It's the study whose authors left themselves open to criticisms from all sides--from science writers and scientists--primarily focused on their strangely lopsided presentation of results--find me the untreated control tumor images in that paper, for example--and their lack of some pretty obvious statistical analyses. 

The study in question took a rat strain that's notorious for developing tumors under regular rat-life conditions, fed the rats genetically modified corn, the herbicide Roundup, or genetically modified corn diets possibly laced with Roundup, and evaluated the various groups for tumor burden, liver and kidney outcomes, and mortality. A total of 180 rats received some treatment in their water or diet, while another 20 just lived their regular rat lives eating a regular rat diet. The open-access paper is available here [PDF].
The authors, including anti-GMO activist/scientist Gilles-Eric Seralini, executed statistical analyses that took an almost global hammering on the Web. The special irony here is this paper by Seralini et al., complaining about statistics in toxicology trials Monsanto conducted, particularly related to power analyses and estimations of effect size. They wrote the paper in response to a 2007 expert panel decision rejecting their own analyses of a GM corn as having any effects related to “treatment” with the corn.
In their current paper, the authors conclude that their data demonstrate an effect of a diet containing GM corn, specifically a corn known as NK603, with or without the addition of Roundup, on tumor outcomes and some other endpoints in their two-year rat study. But they may have overlooked some other factors that influenced their results.
Were the diets even different? Maybe not 
For diet to be the culprit here, the diets themselves would have had to be different. One report suggests that the lab chow the authors used itself might have contained GM corn. The authors stated as much in a previous study of NK603 and two other GM corns, observing that the study offered “no data … to demonstrate that the diets fed to the control and reference groups were indeed free of GM feed.” In their current work, they don’t mention this comparison at all; instead, after chemical analyses, they say that “for the different corns and diets, the study of the standard chemical composition revealed no statistical difference” and describe them as having been classified as “substantially equivalent.”