The Dumbest Mistake a Hero or Heroine Has Made?

Well, I just read a book in which the heroine smears mud all over herself to "mask her scent" from a shifter. Except it's not mud, it's horse manure. That's a pretty dumb mistake in my books.

The one that really irks me though is the "I'm not going to talk to you about this misunderstanding instead I'm going to leave and throw myself into the arms of another man/woman" trope. I know that it's how conflict is built up and that the resolution will be awesome, but if the hero/heroine would've just communicated with their significant other there wouldn't be a problem and heartache would be lessened. Dumbest. Mistake. Ever!

Oh, that happens so very often in romance books, it's quite hard to pick just one example! I actually don't mind if that huge mistake serves the plot, but it will definitely have me roll my eyes, and in some case put the book on the DNF pile, if the said mistake is totally out-of-character or is used as an artificial plot-line.
For instance, I read a book whose title I can't remember and whose entire storyline was based on the heroine refusing to confess the hero that she slept with him and she's pregnant with his child, and the hero refusing to confess the heroine that he keeps dreaming of the mysterious woman he slept with one night he was drunk and can't remember her identity. Basically : the H/h are in love, the h is pregnant with the H's child, the H marries the h thinking she's carrying another man's baby, the h is haunted by the passion she shared with the H but won't tell him, the H feels guilty for being a bad husband because he desires another woman (or so he thinks), the H is mad as hell when he finally learns the truth and they yell awful things at each other but they work on it and get their HEA. To put it, it was quite much ado about nothing.

I appreciate that you used a still from "Atonement" because that was one of the biggest parts of the book/movie that pissed me off.

And I agree with @Lafka, as you can't pick JUST one. In almost every single book I've read recently, there's some version of "Oh no being with you will get you killed or hurt your or both of us because my past is catching up to me, even if I didn't know it was there, so I must leave you now and make up some shitty excuse as to why because if I tell you, you'll ruin every argument I have."

If anyone here has ever seen the movie "Once" -- GREAT film, amazing music... and so heartbreaking in the end. Technically, yes, everyone gets HEA but not the one you want/expect. That's a great example of this. Finding your soulmate, and letting other things come in the way because it's logical.
Granted, if it were logical, I'd probably find a way to screw it up too, I just don't get that chance very often.
-sigh-
I'm a believer in full honesty and, though it will never happen, if more people could be logical enough to just LISTEN to the honest truth and give it time to think it over with an open mind, I think there'd be a lot less discord in the world.
It'll never happen but I'm a dreamer.

My favorite dumb moments come from old Harlequins.

"Love by Degree:" heroine (who is a college student) moves out of her house WITHIN THE SAME DAY after she sees that her live-in boyfriend drove his fancy car to work instead of his beater truck. Because his old girlfriend was the one that rated the Porsche, but he mostly takes her around in his old truck....Guess what, turned out the guy's truck WAS OUT OF GAS THAT DAY. I also laughed at the idea of a college student immediately finding a new home within the same day.

"No Sad Song:" opera singer holds grudge against her big shot agent because she thinks he gets romantically involved with his clients (one of whom was her college mentor, the girl died in an accident after having written a letter saying "Piers says he will never love me."). So she gets involved with the guy and then leaves him at the aisle for REVENGE. Right-o, great idea to screw your career for the sake of a dead girl you hadn't talked to much in the last few years before she died, certainly not enough to find out that "he will never love me" is referring to the girl's actual boyfriend rather than the agent.

Ok, so my personal pet peeve is anytime the author has the main characters, the heroine in particular, act like a complete moron. Even in my fantasy, I want a little realism in how a character might react.
I think one of the worst Harlequin's I ever read had the following plot: a girl's fiance dies only for her to see him sometime later. It turns out, he was an engaged to her as a ruse to find out if she was actually a murdering pyscopath working for her father (who she doesn't really know is a pyscopath) and while he fell in love with her sort of, he's still not sure she's not evil. All of this was on her sister's orders, who despite being a cold bitch, the h was always nice too. H forgives almost immediately. Really? If I found out I was betrayed by both my sister and my fiance, who was only my fiance under orders, I might have more than a minor reaction. Bleh.
Of course, there is also the famous, h who is so tough and independent that she'll ignore all good advice and leaves whatever safe place she's in, only to get into trouble. Inevitably, this happens several times through the story, as she doesn't learn from the first time. To me this doesn't denote bravery, but arrogance and stupidity.
O.K. Those are my rants. Sorry to bore. :O)

The heroine in a historical is being pressured to marry a nasty old man. Just in the nick of time, she is singled out by a handsome young man, and after a couple of kisses in back gardens, she blurts out her problem to him, hoping he will offer to marry her instead. The hero jumps away from her, confessing that he's already married, and when they see each other years later, after their respective mates have passed away, she lets him off the hook way too soon. Sorry, didn't work for me. No matter what the extenuating circumstances, he had no business leading on young girls in dark gardens.