1) Years ago I started a small side-business with a few colleagues. It was a web-based service for a very specific group of clients. Best of all: The idea had already been validated. We had our first paying customer before we even built the product. And we quickly found a few other customers. But after a few sign-ups things were not looking so good anymore. Even though our early adopters were well known in the industry, other companies would not follow. Everyone we talked to loved the product, but as we had to learn the hard way nobody was willing to pay for it. Some even went so far as to implement possibly illegal solutions instead of paying us a few bucks.
Lesson learned: Even if you talk to a few possible customers, question hard if those really are representative of your target audience. Also, liking a product/idea is not the same as being willing (or able) to pay for it.
2) A while back I wanted to find out if there would be any commercial interest in an app I had already built. So I set up a landing page and collected email addresses. Since I had already built the app I could easily hand out demo accounts for every interested party. I quickly collected around 60 addresses and set them all up with a demo account. Only a single person ever logged in at all! I sent out reminder emails but did not get a single response. To this day I do not fully understand what went wrong.
Lesson learned: Newsletter- or Beta-signups from your landing page is NOT the same as validation for your idea.