From the AP via AOL's news portal:
The FBI says it sometimes gets the wrong number when it intercepts conversations in terrorism investigations, an admission critics say underscores a need to revise wiretap provisions in the Patriot Act.
The FBI would not say how often these mistakes happen. And, though any incriminating evidence mistakenly collected is not legally admissible in a criminal case, there is no way of knowing whether it is used to begin an investigation….
38,514 untranslated hours [of recorded calls noted in a Dept. of Justice inspector general's report] included an undetermined number from what the FBI called "collections of materials from the wrong sources due to technical problems."
Spokesman Ed Cogswell said that language describes instances in which the tap was placed on a telephone number other than the one authorized by a court.
"That's mainly an instance in which the telephone company hooked us up to the wrong number or a clerical error here gives us the wrong number," Cogswell said.