A few days ago, my iPhone did something most annoying: the Messages app icon kept showing showing "1" when I had really read all my SMSes and iMessages.
I just couldn't find that "unread" message. It was driving me bananas.
You can imagine what this does to someone who has even the mildest of bit of OCD.
In the end, the way I solved the un-findable unread message on my iPhone was to get Siri to read my unread messages, and then I located and deleted that buggy message.
Boom, the "1" was gone. At least Siri is good for something.