Most startups don't fail because of bad execution. They fail because founders build on assumptions they never validated. Here are the 5 most dangerous ones:

1. "People will pay for this"

The most fatal assumption. You've validated interest, maybe even got signups, but interest ≠ willingness to pay. The gap between "I'd use this" and "I'd pay €19/month for this" is massive.

How to validate: Run a smoke test with actual pricing. Create a landing page with a "Buy Now" button before building anything.

2. "We can acquire customers cheaply"

Every pitch deck shows optimistic CAC projections. Reality hits when you discover your target audience doesn't hang out where you thought, or that reaching them costs 10x your estimates.

How to validate: Run small paid acquisition tests before scaling. €500 in ads can reveal your true CAC.

3. "The market is big enough"

TAM slides are fiction. Your actual serviceable market is usually 1-5% of what you present to investors.

How to validate: Count actual potential customers by name. If you can't list 100 companies or 10,000 individuals, your market might be smaller than you think.

4. "We'll figure out monetization later"

The graveyard of startups is filled with products people loved but wouldn't pay for. If you can't articulate how you'll make money, you don't have a business.

How to validate: Talk to 20 potential customers about pricing before writing a line of code.

5. "Our technology is our moat"

Unless you've invented something truly novel, technology is rarely a sustainable advantage. Competitors can copy features; they can't copy distribution, brand, or network effects.

How to validate: Ask yourself: if a well-funded competitor copied our product tomorrow, why would customers still choose us?

The Antidote

Use Devil's Advocate's Assumption Mapper to surface these hidden beliefs and validate them systematically before they become expensive lessons.

5 Fatal Assumptions That Kill Startups Before Launch