Season 13, episode 2 of The Amazing Race: The detour is a choice between a beach task and going to a shipyard to track down a specific container. Kelly & Christy chose the former, and after completing the task reread the wrong clue, which told them to find the container. You'd expect: That the words "container" and "yard", the apparent lack of such things, or at least several other teams coming and going in front of them without making any effort to find a container would tip them off really quick that they might have read the wrong clue. Or hell, just realize that something is wrong and reread the clue to check. Instead: Kelly & Christy look for that container in the water, on the sand, they use their hands to dig for it, they decide that all the other teams giving them weird looks and leaving are just "giving up", and the fun goes on and on and on... Additionally: This is only the worst of Kelly & Christy's mistakes. They managed to make a bonehead move almost every leg.
Season 1, episode 9 has even more mind-boggling examples. The first one is delivered by Team Guido: they've managed to beat Nancy and Emily to the Fast Forward, an in-game device that allows a team to skip all other tasks and proceed straight to the Pit Stop. You'd expect: Team Guido to hop on the next passing cab or train to the Pit Stop to arrive there in first place with a good lead on the other teams. Instead: They decide to rest in a hotel, confident that their lead is big enough to lounge around for a couple of hours. This causes them to arrive at the Pit Stop in dead last place. Yes, behind even Nancy and Emily, the very team they beat for the green pass. Clearly, they'd never heard the fable of the Tortoise and the Hare.
The sole reason Team Guido wasn't eliminated in the same episode is because Nancy and Emily suffered from an even bigger What An Idiot moment. After failing to get the Fast Forward, they have to return to completing the usual tasks. The Detour gives them a choice between searching streets for a specially-marked private car to drive to their next destination or take a slower-moving bus to the same destination. They opt for the first choice, which while relying heavily on luck, is understandable enough. Unfortunately, luck is not on their side, and they're on the verge of giving up after several minutes of fruitless searching. You'd expect: Them to do what Kevin and Drew did earlier in the episode and switch choices. It isn't too hard to find a bus. Instead: They give up entirely on the Detour and take a taxi to the destination. Even though they arrive hours ahead of Team Guido at the Pit Stop, they're penalized 24 hours for failing to complete the Detour, which is enough to eliminate them. Since Team Guido was the team everyone wanted to see eliminated and Nancy and Emily the ones everyone wanted to see come out ahead, the latter's gaffe was all the more heart-wrenching. The urge to shout at the TV screen, "Just take the damn bus!" was overwhelming.
Separated Couple Tara & Wil were by far the smartest and most well traveled team on Season 2. The only hiccup in their game was their physical skills. You'd Expect: Not making an alliance with the physically strongest team in the game, a team with no communication skills, no travel experience, and not that bright to boot, and dragging them into the Final 3 when they probably would have been dead meat otherwise. Instead: They do just that, making an unnecessary alliance with Chris & Alex, who proceed to pass them up on the run to the Finish Line, stealing the the million dollars right out from under their noses.
On Season 10, the Cho Bros., Erwin & Godwin, take this even further by making the "Six-Pack" Alliance with two other back of the pack teams. You'd Expect: The physically fit brothers to ditch the dead weight once they got into the top 6. Instead: They sit around at tasks, after already finishing them, waiting for their alliance members to finish. Unsurprisingly, they finish 5th.
Season 16, episode 9. Of the 5 teams left, Brent & Caite are in second place at the U-Turn, having the good fortune to have two strong teams behind them, each one having three first-place finishes on their track record up to that point. You'd Expect: They U-Turn one of those two strong teams. Instead: They U-Turn Carol & Brandy for "being rude" (yes, U-Turns do get used emotionally), Carol & Brandy being arguably the weakest team out of those behind them, with not a single first place finish under their belt. This basically meant Brent & Caite wasted their U-Turn to help their biggest rivals while shooting themselves in the foot.
Possibly the dumbest use of the U-Turn was in Season 12, episode 8. Kynt & Vyxsin were saved by a non-elimination leg, requiring them to do an extra task in addition to the regular leg. Naturally, this would put them behind (even if they don't know the precise rankings), and when they get to the U-Turn, the only competition they've seen was Nathan & Jennifer. You'd Expect: They U-Turn Nate & Jen. Even if they finished the task before Kynt & Vyxsin, they're still the safest bet to prevent the possibility of fighting to avoid last place. Instead: They U-Turn Nicholas & Donald (a team that turned out to be ahead of them), based solely on their placings in previous legs. This costly mistake along with Kynt forgetting the receipts in the next task sealed their fate in that leg.
Season 22, Episode 2. Idries and Jamil have an intense fear of water and got to the detour involving going underwater. You'd Expect: Seeing what the detour was (dive down in snorkle gear - Pick A Pearl - or wear a diving helmet - Take a Trunk - and go underwater), the twin doctors would opt for the Take A Trunk detour. Instead: The brothers attempt Pick A Pearl, let their fear overcome them, argue about taking the 6-hour penalty, attempt the Pearl roadblock again, then opted to switch detours, which at that point, they blew an hour and a half lead over two teams and got eliminated.
Season 22, Episode 4. After getting horribly lost and struggling at the Detour, Jessica & John arrive at the roadblock last holding an Express Pass and see only Chuck & Wynona there. You'd Expect: John listens to Jessica and uses the Express Pass, considering she wanted to use it during the leg in Indonesia because she knew they were way behind. Instead: John blows her concern off again, and says that he can beat Chuck & Wynona at a luck-based task, in which you can't say you can beat another team. Jessica stands by and watches Chuck & Wynona check in at the Pit Stop, ensuring that Jessica and John get eliminated.