I got a racquet back from the stringer that they'd cracked accidentally (let a rookie do it) while stringing (made good on replacing it without any problems).
I knew right away - just 'double tapping' a ball to pick it up off the court, I could feel that it wasn't solid anymore.
I wish the OP had taken a quick pic and posted before sanding too. @Kinetic is right - 21st century. New word for the day: ubiquitous.
"I jump in it!" Russian Billionaire, DirecTV Commercial...