A handful of C projects I have seen use magic numbers in allocated structs to prevent use-after-free and other memory bugs[0]. Basically, in this case, when the ref count hits zero and the struct is freed, the magic is zeroed and any further access will be stopped. The author makes no reference of this, so I guess this isn’t a widespread safety pattern?
[0] https://github.com/varnishcache/varnish-cache/blob/4ae73a5b1...