In The Bible, Delilah asks Samson how to take away his Super Strength. Samson tells her that he can lose his strength if he is bound with new ropes or if his hair is braided, both of which are lies. Delilah tries both of these, and summons Philistines to attack him, and fails. Then, after she tries to take away his strength twice and Philistines showed up to attack him both times, Samson tells her to cut his hair, which works. Memorably pointed out by Orson Scott Card's character Alvin Maker.
Then again, Samson is established as not the sharpest knife in the drawer to begin with. Notice that his thoughts even the morning after his hair was cut were "Philistines? Time to whip their butts again! Wait, why am I so weak... OW! MY EYES!!"
Abraham is traveling through Egypt with his lovely wife, Sarah in tow. He fears that the Egyptians would kill him and take her away because of her beauty. So, what does he do? He stuffs her into a box. He didn't stop to think that maybe, just maybe that box would, you know, have to pass through customs. Then when she is discovered, he tells them that she's only his sister, which results in Sarah being taken into the Pharaoh's harem. The real icing on the cake? This happens twice!
And then the *exact* same thing happens to his son Isaac!note In fairness to Isaac, "Hey, did I ever tell you about the two times I almost got your mom raped by the king?" is among the more unlikely things for a father to say to his son.
Probably not the only case in Greek Mythology, but the biggest: Rhea fooled her husband Kronos from devouring little baby Zeus by giving him a stone in diapers. To be fair, she did get him drunk first.
Another notable moment is in (one version of) the myth of Heracles and the centaurs; when Chiron—Heracles' tutor and one of only two good and wise centaurs—is accidentally killed by one of Heracles' poisoned arrows, Pholus—the other good and wise centaur—picks up the arrowhead (in a moment of uncharacteristic foolishness) to marvel at its ability to kill so great and strong a one as Chiron, gets cut on it, and dies from the poison soon after.
In Norse Mythology, Frigg went on a pilgrimage throughout the world and extracted a promise from everything in existence that they would not harm her beloved son Balder. The other gods even made a game out of it, putting the amused Balder in the middle of a circle and throwing things at him just to watch the things dodge him. That's not the Idiot Ball; that's cute. The Idiot Ball shows up when Loki disguises himself as an old woman who manages to get Frigg to explain that she didn't get this promise from one thing - mistletoe, because she deemed it "too young" to be bound to such a vow. He immediately gets mistletoe and tricks Balder's blind brother into throwing it at him, resulting in Balder's death. Frigg, you twit, why would you tell anyone about that, especially knowing that there's a nasty trickster god running around who's really good at disguises?
Any time anyone listens to Loki immediately gets the Idiot Ball. It's somewhat justified in that he's the god of lies and it's in his power to convince people.
It's also justified in that in some myths, he's on their side — before he really goes bad, he's a bastard, but he's their bastard. He does manage to get back Thor's hammer after it's stolen by giants, for instance.