For eg. Anonymous hack a child porn site and dump the database, which they send to police. I can't see an easy way how the police can use that information without it coming up in court as to how they got the lead in the first place. If they were to subpoena that persons email account or apply for a search warrant, the probable cause would need to be something that the police discovered independently.
The database would be a good source of leads, which would have to be worked separately.
It is very possible for something to go wrong. Everybody needs to know about Operation Ore:
http://en.wikipedia.org/wiki/Operation_Ore
US feds sent a database of users of an online payments service that was servicing child porn sites to police in the UK. What was lost in the communication, or what the US feds didn't know, was that the same payment service was also being used by legal or legitimate adult sites.
The UK police treated each user as a child pornographer, and made 3,700+ arrests out of the 7,000+ suspects. Most of these people were innocent, and the dragnet caught up teachers, lawyers, police officers, etc. and resulted in 33 suicides. It took these people a very long time to have their names cleared, and most have never recovered.