The photo below appeared on a canyoneer's Facebook page. It very graphically illustrates why you should never put your fingers inside a bolt hanger. A fall while your finger is in the hanger can result in serious injury or, more commonly, loss of the finger.
A guest from Nepal experienced a near amputation of the tip of his right ring finger in Arizona when the rope below his rappel device whipped up and wrapped around his finger just as he was dropping off the end of the rope into a pool of water.
Let's be careful out there.