I have talked in this space before about how I watch a bunch of cop shows, largely because I watch them while working out. This has advantages (pacing! hurrah pacing!). It also has disadvantages, because dang, are some of the things paced the way I need them to be…kind of obvious, honestly. It’s like you can see the places where they said, “[Find motivation for character here],” and then never did a search on brackets. Except that I’m not convinced that they did. I’m not convinced that in every case there was someone saying, “Uh…that motivation makes no sense.”
Here’s the thing. It’s not that smart people don’t make stupid mistakes. For whatever axis of “smart” you have decided is important in this consideration, you can come up with obvious, boneheaded mistakes that people with lots of that kind of “smarts” will make.
BUT THEY’RE NOT RANDOM MISTAKES.
If you’ve established that a character is both street-smart and good at math, having them decide to go into debt to a loan shark with no known plan of repayment is so far out of character that you have to seriously jump through hoops to justify it. (Yes, actual example.)
That same character might underestimate an opponent’s competence in a number of areas. They might rely on contacts who didn’t come through this time. They might do any of a number of “dumb” things. But for heaven’s sake, make them dumb things that fit. You only get so many foolish choices without it looking like you’re making things too convenient for yourself, or without losing sympathy for the character, or without undermining their characterization as smart. There are all sorts of failure modes here, and you don’t have to give your character perfect decision-making skills to dodge them.
Something that is helpful here: if you have an idea of what a small characteristic error looks like for your character, you can seed that to ramify into the larger ones later, so that a reader doesn’t say, “They’d never make that mistake!” But it does have to ramify throughout, or it doesn’t work.