The US Department of Homeland Security last Friday accidentally shut down 84,000 websites — most of them the domains of private individuals and small businesses — during a mangled operation to seize several child pornography outlets. The gaffe took several days to fix, during which time the affected pages were redirected to a menacing warning that their domains had been seized as part of a Homeland Security investigation.

“Advertisement, distribution, transportation, receipt, and possession of child pornography constitute federal crimes that carry penalties for first time offenders of up to 30 years in federal prison, a $250,000 fine, forfeiture and restitution,” the banner broadcast, wrongly associating thousands of law-abiding web users with child pornography.

Their sites are all back up now, and Homeland Security has gone on to announce and celebrate the seizure of the 10 pornography domains it meant to target in the first place. But the error suggests how easily — and accidentally –– innocent Internet users can get caught up in the hunt for actual cyber criminals.

The incident has also alarmed civil liberties groups already wary of proposed Internet “kill switch” legislation in the US Senate, and the collateral damage that could come from giving government too much control over cyber space. Homeland Security, meanwhile, has been conspicuously silent about the mistake.