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Last Saturday night, while cleaning the house, I was talking to an old friend. We agreed to have dinner on Sunday. Twenty-four hours later, I was surprised when my old friend called. "I'm almost there," she said.

"Where?" I said.

"The restaurant. Where we're meeting."

"Oh no," I said, looking down at my pajamas, which I was not only already wearing but had also splattered with the spaghetti sauce I'd just made. In a split, heart-sinking second, I knew what I was going to have to say, because I'd already said it so many times this year that it was laughable—and humiliating. "I forgot. I'm sorry. I don't know what happened. I didn't used to be like this."

That last phrase is one that so many of us say to ourselves. We didn't used to send our critical business email to the members of our mommy group. We didn't used to forget to add the eggs to the banana-bread batter. We didn't used to put antifreeze in the oil tank of the car—or commit any of those other mindless mistakes that now affect our lives.

This is why I turned to Sendhil Mullainathan, a professor of economics at Harvard. Though an authority in behavioral science, just last week this illustrious thinker forgot his glasses in his motel bathroom—a mishap that not only supports his theory about why all of us keep slipping up, but also provides some insight into how to prevent doing so.

Step 1: Realize It's Not You

Let's start from the beginning of Mullainathan's story. In order to start his job teaching this fall at Harvard, he had to move from New York to Boston. Because he had a small apartment, he allotted 24 hours for the project. The day of, he spent from 8 a.m. to 6 p.m. supervising the movers and packing. From 7 p.m. to 10 p.m., he drove up to Boston in the full van. Then he parked the van, spent the night in a hotel, woke up at 8 a.m., brushed his teeth, washed his face and left his very-much-needed glasses on the counter.

Every time we do something like this, Mullainathan says, we think, "I'm stupid!" Mostly, we believe this because we are convinced that we're the only ones who could have done it. There is no one else to blame. But (a) you're not stupid, and (b) feeling stupid is not the problem and will only cause you to feel discouraged and, frequently, to make more mistakes.

Step 2: Understand When You're Drunk

So what's going on? Though you might not realize it, you might well be "drunk" in terms of your brain's ability to process information. Let's say you've been working on an intense project—moving apartments or finishing a big presentation for work or finishing the fundraising plan for your child's school. In order to complete it, you focus on the project to the fullest extent possible, putting aside anything not related to its responsibilities.

Next: Know when you're going to make mistakes