Via Staffan's Personality Blog, here's an article from a Swedish (ahem, sore loser, ahem) newspaper accusing PISA of using fabricated data from Slovenia, Italy, and the United Arab Emirates. The charges don't involve students, but high school principals. The principals were supposed to fill in a 184 question survey for the Nosey Parkers at PISA, but there is evidence that dozens of principals just cut and pasted somebody else's answers, which wouldn't be hugely surprising with a survey that is 184 questions long.
A general problem with comparing results of countries in international tests are differing levels of motivation. It's remarkable how plausible the PISA results are in general considering how much this factor is likely to vary from place to place and time to time.