[spingsprong]

And this is why we landed on the moon.

Seriously. The Manhattan project involved 130,000 people at its peak and lasted four years, and it leaked.

The Apollo program involved 400,000 people at its peak and lasted eleven years.

[Houshalter]

Ditto for all conspiracy theories that require massive numbers of people to be involved. Notably many 9/11 conspiracy theories.

[axman6]

Would 9/11 really need that many people to stage? The actual plot was executed by a fairly small number of people; surely faking it wouldn't take more than an order of magnitude more, especially when you control much of the process of investigation.

I don't believe it was a conspiracy, but I also don't think that there's a fundamental reason why it definitely couldn't have been.