Patch for Nmap 4.20RC1 (and some 4.20ALPHA versions) that adds a runtime option to toggle the sending of packets with bad TCP/UDP checksums.
You can download it from the following link: https://packetstormsecurity.com/files/download/52433/nmap-runtimebadsum.patch
Source: https://packetstormsecurity.com/files/52433/nmap-runtimebadsum.patch.html