"I was in the classroom walking from student to student while they were working on a Latin test when I saw that one boy was stuck on a question. I put one hand on his shoulder and said: 'Just move on - go to the next thing.' And he looked up at me and said: 'I can't.' So 10 minutes into the test, he was done."
The incident might not have had the impact it did on 45-year-old Lahey had her own nine-year-old son not been grappling with the issue of tying his shoelaces at the time.
Indeed, just a few days earlier, mother and son's feelings of helplessness had given way to anger and tears.
"I looked out of the classroom window at my son, who attended the same school, and my worlds collided: I was both a parent and a teacher, with all the fears and frustrations of both," she says.
Well before then, Lahey had found herself in the curious position of having to "defend" her students from their parents. "I wasn't telling parents about things that were going wrong in school because I didn't want to subject the kids to their parents' ire," she says.
"My worry was that these parents would come down so hard on their kids that they wouldn't be able to function at all."
Today's overprotective, failure-avoidant parenting style, Lahey believes, has undermined the competence, independence and academic potential of an entire generation.
"What we're seeing now are almost adults who are so freaked out by real life that they are practically incapable of dealing with college. In all our attempts to get them to further education, we're breeding kids who don't know how to write an email to a teacher and will ask their professors to reschedule things around their vacations - and it's really detrimental to their lives."
Whereas most child-rearing guides are read through parted fingers and intermittent bouts of self-flagellation, The Gift of Failure makes it clear from the outset that it's not too late.
Rather than adopt the smug, didactic tone of so many parenting experts, Lahey lays out her own mistakes in all their banal and small-time glory - and explains how to kick-start the admittedly laborious process of back-pedaling.
"Parenting for dependence simply doesn't work: the child will sacrifice his or her natural curiosity and love of learning at the alter of achievement," she writes.
"Intrinsic motivation" is the holy grail of parenting, Lahey insists - and that means ditching bribery in order to allow children to engage with their own education for the sake and love of learning.
"The problem is that bribery doesn't work as a long-term system. Children will reach a point where they don't care enough about rewards to do what you want. So intermittent rewards work better than a reward every time."
Verbal bribery and cajolery can be just as detrimental, Lahey goes on.
"Telling your kid they're 'so smart' when they score well in a test is still a judgment of sorts," she says.
"Of course kids get that we love them more when they bring home the paper with the big stars on it, but research shows that the most damaging thing you can do is predicate your love on a child's performance - or worse, withdraw your love as punishment."
Basically, we felt that if we told our kids they were wonderful enough, then we could create this force-field of wonderfulness that would somehow repel the laser-blasts of mean comments throughout life
We're allowed to be disappointed, Lahey points out.
"By saying, 'I noticed that you were cramming for the test the night before, so maybe that's not the best way to go about it', you're encouraging the child to think 'how can I get better at this?', rather than just berating themselves for 'failing'."
Allowing children to embrace autonomy early on (from the age of three or four, Lahey says, they should be able to make their own beds, put their socks in the laundry, and learn to clean up their own spills) will teach them that self-reliance "feels great".
Much of Lahey's advice may sound like common sense, but in a world dominated by complex and conflicting philosophies and "sciences", this is a valuable commodity. Lahey believes that the self-esteem movement of the late Sixties and early Seventies has been one of the most damaging "philosophies" out there.
"Basically, we felt that if we told our kids they were wonderful enough, then we could create this force-field of wonderfulness that would somehow repel the laser-blasts of mean comments throughout life," she says.
"In some ways we're still stuck in that era," Lahey adds."Hyper-awareness of kids with differences has in many ways been fantastic," she concedes.
"But when I write about behavioural problems, I get a lot of emails from parents saying: 'How dare you! My kid is different'. We've got to a place where we feel that every child is special in their own snowflake kind of way, and that we can't make generalisations because they're all individuals. That has created a dangerous situation where the kids themselves think: 'I'm special, so this doesn't apply to me'.
"Social media has only exacerbated the problem.
"Parents are getting more and more of their own sense of self-worth from how their children perform, now that our lives are laid out on Twitter and Facebook. And, of course, everybody's lives look great on the page: nobody is going to show us all the crap. But parenting is a whole lot of crap," says Lahey.
I wonder whether that should have been her book title. Because the atychiphobia so much of us persist in seeing as a quality rather than a fault is surely as much to do with our own fear of failure as the desire to see our kids succeed.
"We have to talk about failure as a life-long thing," Lahey sighs. "Because it's not like you hit adulthood and suddenly you're good at everything."
Far from it. If anything, you just keep discovering new things to fail at.
