It is true that in most cases if police have obtained a search warrant it is likely that any evidence they obtained as a result will be admissible in court. In order to obtain a search warrant police must have probable cause and state with particularity what exactly they will be searching and what items they are searching to the magistrate. Most of the time this will be valid; however there are cases in which evidence obtained as a result of the search warrant cannot be admitted to court.
There are three main situations in which any evidence obtained because of a warrant would be considered to be obtained illegally. This would be: 1.) If police either lied or recklessly disregarded the truth when obtaining the warrant; 2.) If the warrant on its face was so deficient that any reasonably trained police officer should know it was invalid; and 3.) The judge or magistrate so abandoned his role in issuing the warrant that it lacks any indicia of probable cause such that any reasonably trained officer should know it was invalid. Basically if the officer lied to get the warrant, if the warrant is not filled out correctly or the officer should know there was not probable cause to have the warrant issued than the evidence that was obtained will not be admissible in court.
There are other variations and circumstances where evidence obtained as a result of a search warrant will not be admissible in court. The situations listed here would make an issued search warrant invalid and thus resulting in an illegal search.
