The opportunity to speak to a friend or family member is often welcome for someone who is under arrest. It can be helpful, too, if it facilitates payment of bail or finding a lawyer. But it also has pitfalls.
A common police tactic is to allow an arrested suspect to speak with a visitor, oftentimes at the police station. The suspect, having forgotten (or not realized) that he is being recorded, makes some kind of incriminating statement to the visitor.
Unfortunately for suspects in this situation, a recording of the conversation with the visitor (or perhaps an officer’s account of the conversation) may be admissible at trial, regardless of Miranda rights. The lesson for people who’ve been arrested is to assume that anything they say while in custody is being recorded.