This article by VerticalResponse CEO and founder Janine Popick originally appeared on Inc.com.

I was talking to our head of technology, Eric, the other day about some engineering costs we’re going to incur with an outside vendor. Eric was looking around at two or three different vendors to do the trick for us. So I asked him about one of the vendors he was assessing. He put a pretty strong kibosh on them. I’d never seen him shut a company down that quickly. What could they possibly have done? He told me, “A clerical error, but a pretty big one.”

So I went on to discover what a simple error cost this vendor, and I wanted to share it with you because now I can see Eric’s point.

Here’s what happened:

• They sent an email marketing campaign inviting him (and only him) to a webinar through email. Great! He bit. It was free and he’d get to learn a bunch more about the company.
• They confirmed with a Google Calendar invite to the webinar where another 60 people from very prominent companies were cc’d.

Ouch! We saw that Macy’s, Gap and Kiva were all interested in learning about what this vendor had to offer, and conversely, they saw that we were interested, too. Not that any of these companies compete with us, but what if a competitor was on that list? Not only that, but everyone now had access to everyone else’s email address, to add to any spam list.

The vendor said that it was simply a clerical error.

The kicker? This vendor could have been a contender for a pretty large sum of revenue from us that now they’ll never see.

If you’ve got internal people on an email and you’re cc-ing a few, that’s one thing. But when you start to “import a list” of outside people, you could find yourself in deep trouble. Has this ever happened to you?

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