If the police have a warrant to search your house, it's all over, isn't it?
No, it's not. Many errors are made on search warrants, and on the sworn affidavits that form the basis for the judge's decision to issue the warrant. Something as simple as a typo giving the wrong street address might make the evidence they seized inadmissible. Sometimes the affidavits are based on intentionally false statments, and if the false statements are critical to the decision to issue the warrant, it can also result in the evidence being excluded.