You've apprehended a suspect. You have 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.

Sometimes overlaps with A Bloody Mess, for which The Ketchup Test might be used for The Reveal. If the object in question is a powder or liquid, the Fingertip Drug Analysis may be used instead.

Examples:

Comedy

One Peter Kay skit related his Crazy-Prepared dad's trouble carrying a small plastic bag of Coffeemate, a milk substitute that comes in the form of a white powder, through foreign customs.

Film

Subverted in Starsky And 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.

Literature

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.

Live-Action TV

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.
- Also, a drug-sniffing dog points to his desk at work, prompting Drew to open the drawer, which contained ribs.
"When ribs are outlawed, only outlaws will have ribs!"
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: Crime Scene Investigation 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.

Video Games

This tends to happen a lot in the Ace Attorney series, mostly in the inverse direction, but there are some normal moments too.

Real Life

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.

The Meddling Kids Are Useless