In a case of a literal Idiot Ball, Xykon wipes out the Azure City paladins by tossing a superbounce ball inscribed with a Symbol of Insanity into their midst, causing them to turn on each other in a tremendous bloodbath of confusion.
When Vaarsuvius decides to take onXykonby himself, it's not nearly as stupid an idea as it might seem due to a Deal with the Devil (and the Demon, and the Daemon) s/he made recently to gain a massive amount of power. Too bad s/he's carrying an Idiot Ball for the entirefight. Used to trigger character growth when it makes him/her realize that simply throwing spells at a target isn't enough to take it down without strategy - soon after, V and a paladin manage to severely inconvenience the Big Bad with a third-level spell and a class feature.
Old Blind Pete believes that coming home while it is in the middle of a massive Thieves Guild operation involving fighting numerous high-level characters he personally betrayed was a good idea. He couldn't even wait ten minutes (literally, the cleric couldn't cast Sending in that time, which has a ten minute casting time).
Author Rich Burlew is also on the record stating that characters carrying the idiot ball for whatever reason is sometimes necessary for good drama: if everyone always made the best available decisions, there wouldn't be much plot, and it would also be unrealistic.
Hazel from Girls with Slingshots goes through an insane level of mental gymnastics in her belief that Jaime's new love interest must be male, despite knowing she's been with girls before. When Jaime tells Hazel her new "boyfriend" is Erin, Hazel (who, admittedly, doesn't remember Erin's name) hears it as "Aaron", even when Jaime repeats it several times, and when Erin herself points out her own name with a meaningful stare. They then go on a double date, where Erin shows up in a bowler hat, glasses, and a fake moustache, and not only does Hazel not recognize her, but has to be told by her boyfriend that Erin is female, after she and Jaime have gone to the restroom together.
"Aaron" and "Erin" are homophonous in many regions in North America. Erin is an androgynous name, also.
A rather in-character Idiot Ball is given to Jade in Jade 6, when she helps with a rather... dangerous-sounding deal. Thing is, she said she'd help, the person really needs it, it SHOULD all go okay, right? (She's shown ALMOST catching herself, but she keeps slipping back.) Partially caused by a massive Hero Ball, as well.
Jim has a lot of idiot ball moments, though, in this case, it's a more integral part of the character, and generally gets used for laughs. The entire plot of the series effectively starts because of such a moment.
The entire plot of the comic is based off of people refusing to let go of the Idiot Ball (usually Jim, but everyone gets a turn). Not to mention the entire point of the comic is to explain the large amount of Idiot Balls in the prequels (see above).
In The Dreamer at the end of issue #10, Alexander offers to take Beatrice to safety to Washington's headquarters, but Beatrice stays put with Alan and Nathan. Alan is supposed to take Beatrice to Washington for interrogation about what happened when she was with Howe - and Alexander could easily have taken her to Washington himself!
This strip from Chaff City features a literal Idiot Ball for sale in a gadget shop - the characters experiment with picking it up, and as the title of the strip states, Hilarity Ensues.
Shiniez. Keen to convince Alan to try more intense bondage with her, Marion did something everyone including herself agrees was stupid, unfortunately for Marion she realised this only at the moment she slipped the knots about her wrists tight without a way to undo them.