I remember hearing more than once from unofficial sources (and from people whose main domain isn't law) that you can't use a recording as a proof at the court, if said recording wasn't taken with the person's consent (or if it isn't a public area with some clear sign of recording devices, e.g. train stations for example).
For example: if I were to hide my phone in my back in recording mode, to record someone during a conversation, while they are saying something I could sue them for.
Now I know there are some cases of exception (e.g. if people are suspecting someone might be taking part to some terrorism acts for example, but that's the extreme case).
In what condition can a recording be considered as illegal, or otherwise, when is it legal to use a recorded material as a proof, even without consent of recording?