SIGSYS
A forbidden system call — nowadays it's seccomp that rings it.
Originally, bad system call: the process asked the kernel for a service that doesn't exist. It stayed nearly mute for decades — invalid syscall numbers are rare in sane binaries.
seccomp resurrected it: filters that list a process's allowed syscalls ring SIGSYS when it tries something off the list. Browsers, containers and systemd use this as a cage — and the core dump says exactly which call the process attempted.
$ systemd-run --user -p SystemCallFilter=~socket -t ./sonda
Bad system call (core dumped)
(the seccomp filter vetoed the socket syscall)