What's the most expensive mistake you've made because you had incomplete information?

I think a lot of bad decisions don't happen because we're careless.

They happen because we don't have the full picture.

A missed detail.
An assumption.
A piece of information we didn't know to look for.

I've become a lot more intentional about how I gather information before making decisions, and tools like FixerFlip have helped me create a more organized process.

But I'm interested in hearing from everyone else:

What's a mistake that taught you the value of having better information?