“With every technology, there is a dark side,” said Hany Farid, a computer scientist at Dartmouth. “Sometimes you can predict it, but often you can’t.”

A decade ago, he noted, no one predicted that cellphones and text messaging would lead to traffic accidents caused by distracted drivers. And, he said, it was difficult to foresee that the rise of Facebook and Twitter and personal blogs would become troves of data to be collected and exploited in tracking people’s online behavior.

Often, a technology that is benign in one setting can cause harm in a different context. Google confronted that problem this year with its face-recognition software. In its Picasa photo-storing and sharing service, face recognition helps people find and organize pictures of family and friends.

But the company took a different approach with Goggles, which lets a person snap a photograph with a smartphone, setting off an Internet search. Take a picture of the Eiffel Tower and links to Web pages with background information and articles about it appear on the phone’s screen. Take a picture of a wine bottle and up come links to reviews of that vintage.

Google could have put face recognition into the Goggles application; indeed, many users have asked for it. But Google decided against it because smartphones can be used to take pictures of individuals without their knowledge, and a face match could retrieve all kinds of personal information — name, occupation, address, workplace.

“It was just too sensitive, and we didn’t want to go there,” said Eric E. Schmidt, the chief executive of Google. “You want to avoid enabling stalker behavior.”