On the first day of Minnesota's recount process, the Coleman campaign challenged roughly 2.3 ballots for every 10,000 cast, and the Franken campaign challenged 2.5, for a total of 4.8 challenges per 10,000 ballots according to figures compiled by Minnesota Secretary of State.
Yesterday, the second day of the process, that challenge rate rose to 3.1 per 10,000 ballots for Coleman, and 3.4 per 10,000 ballots for Franken, for a challenge rate of 6.5 ballots per 10,000.
And now, on day three, according to the Star Tribune, "a rash of challenges has marked today's recount action in Ramsey County [St. Paul and outskirts], with observers from both campaigns questioning voters' intent far more widely than they did the first two days."
It is hard to know whether this phenomenon is statewide or occurring in Ramsay County only, which had had an usually low rate of challenges over the first two days of ballot counting. Nevertheless, given that there are very few disincentives to challenge ballots, it is not surprising that something of an arm's-race has occurred as both sides learn from one another's behavior.
EDIT: The Uptake has video of 10 ballots identified by Franken that were challenged by the Coleman campaign simply because voter voted both for John McCain and Al Franken! Uh, in case you were wondering, the campaigns are no longer leaving any stone unturned.
The disincentive to challenge ballots is precisely this sort of thing -- challenges that look so ridiculous that they'll weaken your ability to take the moral highground. If the Coleman campaign is on the ball, they'll be ready to show similar frivolities from the Franken campaign as well. The Uptake also reports that the Secretary of State is also taking under advisement a proposal to make all challenged ballots available for public viewing on a website.