Innocent people have been caught up in criminal investigations because of snooping blunders.
Individuals were wrongly arrested or visited by police after errors in attempts to access communications data.
Other mistakes led to search warrants being executed at addresses unconnected to the inquiry.
There were also instances in which blunders meant welfare checks on vulnerable people were delayed.
Watchdog, the Interception of Communications Commissioner, investigated 23 serious communications data errors in 2015.
The number includes mistakes made either by law enforcement agencies or communications service providers, and the majority are understood to relate to child sexual abuse investigations.
Communications data covers the who, when and where but not the content of an email or text message.