You may have run into the story of a fireman charged with burning down his own home. They charged him because his Safeway Club card records showed he had purchased the type of firestarter that was used in the arson on his house.
Sounds like a good case? Problem is somebody else confessed to the arson. He's now a free man.
People often wonder why privacy advocates get up in arms about things like the Safeway database. I mean, how can it harm you, especially if you're not doing anything suspicious?
The problem is that police are attracted to the evidence that is easy to find. But when databases become more and more comprehensive, the chance that they will contain something interesting grows.
In an old-time investigation, finding receipts for the firestarters would be a major clue, and mind convict somebody. That's because searches of what you bought weren't so easy. If you bought the very tool used in the crime, and it was prominent enough that they found it, it looked bad for you.
But the cops aren't aware they are falling into one of the traps of bad science. When you have a lot of data, you can always find something that matches what you are looking for. When you find it, your intuition tells you "this is too strange to be coincidence." But in fact math tells us that it is. That's why you must never start with the conclusion and dig around in a big pool of data looking for evidence of your conclusion. Good scientists have known not to do this for years. Cops haven't.