In the original Super Mario Bros. 1, The Evil Bowser, King of the Koopas has booby-trapped all eight of the Mushroom Kingdom's strongholds with flame arms, and numerous pits of lava & nothingness to prevent any intruders from breaking in and rescuing the Toad/Princess who is imprisoned there. At the end of each stronghold, Bowser(?) awaits any intruder who dares get through all the traps to challenge him directly, on top of a bridge over lava that can be opened by a lever behind Bowser. Handy for sending any living thing down to their doom should they be on that bridge.
You'd Expect: That Bowser just flip or kick the lever Right Bloody Behind Him in order to guarantee a quick and easy death the moment Mario shows up.
Instead: He just bounces up and down blowing fire at Mario while still on the bridge the whole time, allowing for Mario to dash right below or jump right above Bowser and hit the switch himself sending -you know who- down his own push-button trap
When Katz meets the enemy Fake Defector Sarah, he's immediately attracted to her. All the adults he talks to warn him about trusting her, because they're still unsure of her intentions and she's still being vetted to find out if her information and intent are for real. When a battle starts, she offers to teach him how to pilot her mobile suit (because he's frustrated with not having his own), which of course means letting her out of the brig and taking her to the mobile suit bay.
You'd Expect: That he would, you know, not do that.
Instead: He does what she asks, and she promptly takes her mobile suit back and escapes.
Jamitov Hymen, who is the leader of the Titans, tries to negotiate to get the Axis fleet commanded by Haman Karn to support him in supressing the AEUG. This is after the first time he tried to negotiate with her, when she tried to assassinate him. He also brings Paptimus Scirocco, a man he doesn't trust.
You'd Expect: Him to take extra precautions against assassination, such as having other guards, or disarming Scirocco, or having a mobile suit nearby. Hell, maybe even not talk with Haman since she stated she was basically only at earth to kill him.
Instead: The talks turn violent and in the confusion, Scirocco assassinates him and blames it on Haman.
In the Strider NES Game, a wounded Strider Cain wakes up.
You'd Expect: Him to stay put while Strider Sheena tends to his wound.
Instead: He runs off.
The Result: He dies at Matic's hand before the last stage.
After Hiryu destroys the ZAIN Terminal in Africa, he returns to the Blue Dragon.
You'd expect: Sheena(A plain A-Ranked Strider) to wait for Hiryu when he heads to Las Angeles.
Instead: She takes off without him.
The Result: She gets fatally wounded by Strider Vice Director Matic and dies.
In "A Clue for Scooby-Doo", the second episode of Scooby Doo, Where Are You!, Daphne, Fred, and Velma stumble upon what looks like Captain Cutler in a sunken ship.
You'd expect: That at least one of them remains outside while one or two of the others take(s) a look at the diving suit.
Instead: They all go in, which results in the ghost locking them in.
In the Angry Birds Movie, the pigs wreck the town while stealing the eggs.
You'd expect: That they either destroy the slingshot they gave the birds or take it away.
Instead: they leave it alone.
As a result: The birds use it to break into the pigs' city to get the eggs back, which ultimately leaves the city in shambles.
In Barney's Great Adventure, the Dream Maker egg ends up at a restaurant after B.J. accidentally tosses it.
You'd expect: That the restaurant sends it to the lost and found.
Instead: It sends the egg to the circus.
At the end of the CGI Lorax film, the environment outside the town has been exposed thanks to a wall being destroyed.
You'd expect: Mayor O'hare to be serious about turning a new leaf.
Instead: He reveals he was kidding and orders the seed destroyed.
The result: He gets impeached and sent flying away.
Jet Force Gemini: In the mine on Rith Essa, you come across Farmer, one of Mizar's minions, while disguised as a tribal.
You'd expect: him to see through the disguise(You were carrying a weapon and Floyd is next to you).
Instead: He falls for it and gives you one of the ship parts you were after.
The possible result: Him getting executed by Mizar for the blunder.
In the Garage episode of the Mr. Men Show, Mr. Fussy allowed Mr. Messy to store a surprisingly clean bike in his(Fussy's) garage, soon after, Mr. Nosy and Mr. Small offer Mr. Fussy to go on a bike trip with them using Mr. Messy's bike(he did get permission.
You'd expect: Mr. Fussy to close and lock his garage so that Mr. Messy can't get in without asking.
Instead: He apparently forgets to do so which leads to Mr. Messy making a huge mess in it.