Quake 1 and NetQuake servers allow remote attackers to cause a denial of service (resource exhaustion or forced disconnection) via a flood of spoofed UDP connection packets, which exceeds the server's player limit.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6871
http://www.securityfocus.com/bid/3051
http://www.securityfocus.com/archive/1/197268