Yes, there have been instances where individuals who were guilty of a crime were acquitted because the police obtained evidence unlawfully. In many legal systems, including that of the United States, evidence that is gathered in violation of a person's constitutional rights may be deemed inadmissible in court. This principle is known as the "exclusionary rule."
The exclusionary rule is a legal doctrine that prevents evidence obtained through illegal searches, seizures, or other violations of constitutional rights (such as the right to remain silent or the right to an attorney) from being used against the accused in a criminal trial. If the evidence is considered tainted or "fruit of the poisonous tree" due to its unlawful acquisition, it cannot be presented in court.
An example of this could be when the police conduct a search without a warrant or without valid consent, or when they coerce a confession from a suspect in violation of their Miranda rights. If such evidence is crucial to the prosecution's case, and it is excluded from the trial, it may weaken or even completely collapse the prosecution's ability to prove the defendant's guilt beyond a reasonable doubt, leading to an acquittal.
While the exclusionary rule aims to protect individuals' rights and prevent law enforcement from using improperly obtained evidence, it can also result in the acquittal of individuals who may indeed be guilty of a crime but whose guilt cannot be proven in court without the tainted evidence. The rule is often a subject of debate, as some argue that it may let some guilty individuals go free, while others believe it is a necessary safeguard to uphold the integrity of the justice system and protect individuals' rights from government overreach.