When police ask a judge for a search warrant, they are required to explain why they have probable cause to search the person or location in question. The Constitution requires it.
To justify a search of someone's residence, for example, it's not enough that the defendant is suspected of a crime. It's not enough for police to have a hunch that evidence of a crime might be found at the search location. They must demonstrate that they have good reason, based on articulable facts, that specific evidence is likely to be found there.
Recently, the influential D.C. Circuit Court of Appeals ruled that police had failed to demonstrate probable cause when they applied for a search warrant. They suspected a certain man of being a getaway driver in a crime, and so they applied for a search warrant to comb through his residence and cellphone.
Warrant in hand, they headed over to the defendant's residence. They apparently didn't find any proof that he had been the getaway driver, but they did find something unexpected. They seized a gun that had been tossed from a window. Unfortunately for the defendant, he had a felony record that precluded him from possessing firearms.
The man was charged with and convicted of unlawful possession of a firearm by a felon. He appealed.
Most people have cellphones, so they're a good place to start searching -- right?
The D.C. Circuit ruled that the warrant was too broad. For one thing, the officers hadn't even verified that the man owned a cellphone before asking for a warrant to search it. This made the justification for the search highly speculative -- they surely had no concrete reason to suspect the phone contained incriminating messages.
"The assumption that most people own a cell phone would not automatically justify an open-ended warrant to search a home anytime officers seek a person's phone," reads the opinion.
The court ruled that the warrant was unconstitutionally granted. Since the government isn't allowed to benefit from violating the constitution, anything gained as a result of the invalid search warrant is inadmissible as evidence.
In other words, the gun the police found was inadmissible as evidence, and the man's conviction was overturned.