You've apprehended a suspect. You have a video of him carrying a large plastic bag filled with a suspicious-looking white powder. This must be the easiest open-and-shut case you've seen all year. In fact, the bag of powder has just come back from the lab identified as...cake mix?
This is a common way to throw a plot twist into a Forensic Drama or Police Procedural - a crucial piece of evidence has been identified as some object, looks like said object, and everybody has been assuming it's that object. Then it suddenly turns out to be something completely different, which completely blows the whole case apart.
It may also be used for comedy, when the true identity of the piece of evidence is absurd and/or ironic.
Similar to the Red Herring, only this is caused by a mistake on the part of the investigators, as opposed to deliberate misdirection by suspects or witnesses.
- Subverted in Starsky & Hutch. One of the plot points is cocaine that drug dogs can't detect; when the title characters bring it in, they're told they've found powdered sugar. It's not.
- In Wilt, after Henry Wilt is accused of murdering his wife, the police search his house and find a lot of what looks like damning evidence, including a cleaver he had used to open a can of red lead.
- In the Discworld novel Feet of Clay a group of "respectable citizens" discover Vimes drunk and unconscious at his desk with a bag of suspicious white power in his drawer... and the Patrician has been recently poisoned with arsenic. Vimes wakes up and quickly eats the evidence. It was just sugar, he hid the actual arsenic that had been planted in his desk to frame him and then faked being drunk.
- In Arthur Conan Doyle's A Study in Scarlet, victims are found surrounded by splashes of blood, including a word written in blood on the wall. Like the CSI example below, it's actually a murderer with a nosebleed. That is, there IS evidence present that helps catch the killer - but the blood isn't it.
- In The Teenage Worrier's Panick Diary, Letty discovers that her boyfriend Basil is actually a criminal who burgled her best friend's house. She then remembers that he left a suitcase at her house. When she opens it, it contains what she thinks is rifles, along with a bag of mysterious white powder. When she finally calls the police a week later (because she had flu and was too sick to do anything about it) they tell her she will not be charged because the "guns" and "drugs" were actually golf clubs and soap powder.
- One episode of NCIS had the team investigating what they believed was a murder caught on video. Halfway in, Abby discovers that the knife that was supposedly used as the murder weapon is a prop knife that drips stage blood.
- She demonstrated the knife was fake by using it on herself. This freaked out everyone that was watching.
- One episode of The Drew Carey Show had Drew's being investigated on suspicion of being a junkie. The white powder on Drew's coffee table is revealed to be powdered sugar from an extraordinary quantity of donuts, and one police officer found a professional-grade scale thinking it was being used to make meth, when Drew was trying to prove that Quarter Pounders weigh less than a quarter pound in order to file a lawsuit.
"When ribs are outlawed, only outlaws will have ribs!"
- Also, a drug-sniffing dog points to his desk at work, prompting Drew to open the drawer, which contained ribs.
"It's like they've never seen a baggie full of eye shadow before."
- Another episode has Mimi stopped by the police, and subsequently being arrested when they discover a plastic bag full of a weird colored powder on her. Turns out it's her Excessive Evil Eyeshadow taken out of its native tin for some reason.
- Once on Matlock, Matlock and Tyler (his private investigator) were investigating a person who had some strange powdery substance delivered to his house every day. Tyler was working undercover as a chauffeur. They met late at night in a dark alleyway for Ben to get a sample for testing. Then the cops came and busted Matlock, since it was a known drug trafficking hangout. Matlock protested his innocence but was arrested. The cops tested the powder - it was a diet formula.
- A CSI episode started with a room with walled covered in spattered blood. Later it was found that the blood had come from a man with a nosebleed, deliberately messing up the walls.
- In the Polish series Zmiennicy, a drug sniffing dog at an airport get excited about a traveller's bag and the border security rip through his possessions searching for the drugs. Turns out the traveller was bringing in a fair bit of sausage in his baggage and the dog has been badly trained and reacts the same way to the smell of meat as he does to the smell of drugs. This presents a serious problem for the border guards since at the time most Polish travellers could be assumed to be bringing in some sort of meat products in their baggage.
- In the Raising the Bar episode "Is There a Doctor in the House", Kellerman's client is arrested after the police find a large amount of white powder in her car. It's laundry detergent, as she was on her way to the laundromat. The cop who arrested her is humiliated and fraudulently charges her with selling a Beat Bag.
- In The Good Wife one of Alicia's clients pulls the cake mix ruse. His motive was to root out which of his lawyers was informing on him. The time frame points to Alicia, but she isn't the snitch (at the time, her phone was tapped by the NSA, which was passing information to the DEA).
- Possibly an urban myth: Three 7th-grade students at a Chicago public school were once arrested and brought to the police station for selling bags of a powder, which they continually insisted was just Kool-Aid. After spending 3 months trying to press charges, the prosecutors finally agreed to test the powder - and it turned out to be grape Kool-Aid mix. Due to the dangers involved in on-the-spot manual analysis of suspicious white powders (read: sampling or sniffing) an arrest will usually be made on the grounds of suspicious behavior or prior suspicion and the arrested will be detained until the powder can be tested - whether in the lab or using special equipment or dogs.
- Played straight in the 2002 Beltway sniper attacks. The snipers targeted people just off of interstates and major highways, to ensure they could get away quickly. Because of he high volume of commercial traffic that used those highways, many eyewitnesses at the shootings reported seeing a "white van or box truck speeding away." The police and FBI ran with this theory, and ignored a report at one of the first shootings about an older blue Chevrolet Caprice. Eventually, other evidence was revealed tying the Caprice to the snipers, at which point the investigators realized that it had been found at multiple checkpoints after each shooting.