Just broken, nothing special. Someone or something snapped it as you can see the rough edges on the reverse pics.
DEFDAM - Definitely Damage Your 1999 Cent is a copper plated zinc cent. Something caused that part to break away. It occurred after it left the Mint. Not a Mint Error.
@JorjeFresco64 This is what actually happens when you have a Die Break.. It's a break on the Die itself which alters the appearance on any coin - http://cuds-on-coins.com We call them CUDs