What's The Best Example Of Someone Underestimating Someone/Something?

[Seil]

Underestimating your opponent. We've all seen it in movies, television, books, whatever. Where the villain doesn't think the protagonist is a threat and sends their weakest goons out to destroy them, or someone gets a good piece of advice from someone who knows what the hell tbhey're talking about, and chooses to ignore it.

The most recent case of this I've seen is in Doctor Who, in the revived series, in Dalek. There's a Dalek running around and The Doctor, who knows that they're pretty much destruction incarnate, tells a security chief a good piece of advice. Which he promptly chooses to ignore.

And then he dies.

Any other cases of characters underestimating the threat?