There's no room for doubt in the hiring process.

That's a mantra that Luis von Ahn, CEO and founder of language learning app Duolingo, lives by.

He meets with every single person interviewing at his company, which currently employs about 70 people.

"The best hiring advice that I've ever got is: 'When in doubt, don't hire,'" he tells Business Insider.

"It is very common that we're in doubt," von Ahn says, which accounts for why the Duolingo team remains so lean.

Von Ahn says that the value of the advice becomes even clearer whenever it's not heeded.

"Every now and then we don't follow that because we think we really need this position filled," he says. "We've been looking for this particular position for months. With this person, we know they have some bad things about them, but we need this position filled. Every time we have succumbed to that it has been a mistake."

Hiring a bad fit is almost always a costly and time-consuming error for any company. A study from the Society for Human Resource Management found that 41% of hiring managers estimate that hiring the wrong person costs the company thousands for dollars, as Business Insider previously reported.

"You spend all this effort onboarding them, and then a few months later things don't work out," von Ahn says. "And things usually don't work out because of the thing that you knew about. You knew what the problem was going to be. You knew that maybe they were going to be a little slow, and it turns out they're really slow. Or maybe they're not going to be very nice. Turns out, they're really not nice."