Characters Who Do Uncharacteristically Stupid Things
Fictional characters—especially the main ones—are generally supposed to be brilliant in some particular field, whether it be investigation, science, crime, or whatever. Why is it, then, that they always make incredibly stupid blunders which any real person with even a fraction of their alleged skill would avoid?
A paranoid cop isn’t going to leave his front door open, allowing a murderer to sneak in while he investigates a noise outside. A bad guy isn’t going to stand around monologuing, giving someone time to get away. Most people know it’s not the best idea to walk down a dark alley.
And then there are the characters who should know things, but don’t. A real hacker is going to know what encryption is. Prison guards know better than to stick their head next to the bars, allowing prisoners to grab them and take the keys.
That’s not to say that characters should never make mistakes. They just shouldn’t be so mind-numbingly stupid. If they’re shown to actually be quite smart—shown, not implied—it makes it far more powerful when they do screw up.