In some precincts, ballot boxes that were to remain sealed and locked up were found opened. In other precincts, the numbers of votes exceeded the numbers of ballots handed out. Errors of math, counting or procedure have turned up at thousands of precincts.
Most of these errors are minor, the PAN points out. They cut both ways. And taken in total, Calderon's aides say, they do not change the outcome.
But some irregularities seem to go beyond "human error."
In Durango, for example, officials opened one box only to find empty envelopes where the ballots should be, according to the Mexico City daily El Universal.
"If this thing is that sloppy, they should annul it," said Mark Weisbrot of the Center for Economic and Policy Research in Washington. "If they were taking a random sample, there is a high probability that based on a recount you could say what happened.
"But since this is not a random sample, no matter what result comes out, there is no way you will really know," said Weisbrot, whose left-leaning think tank did a critical study of how electoral authorities counted the ballots in the days right after the vote. "There are a lot of big questions."