Just a murder
One of the customers of the tiny ISP I was working for had been murdered. My phone appeared in the call logs, so I was cordially invited to the police station to ask me about my whereabouts. They asked if I had had a call with the victim, but I totally forgot I did, and I said no. That prompted them to call me in again (somehow they didn't know about the call the first time, so I got to walk home across town only to be asked to walk back), and be accused of lying. Then it dawned on me that about half a year before I received one call from the victim about his Internet access. We never met in person. And when I mentioned that to the cop he appeared surprised that it was so long ago.
I'm guessing they were experts in murder investigation if it took me two trips to the station for them to find out those crucial details.