"Write a program that halts due to error." Doesn't that mean that almost every programming language would be a solution? (E.g. in your main example, Java would be a legitimate robber answer since it also fails.)

@Moshi Very true. I do like the concept, though, and would like to implement it somehow into this format. SIGSEGV would limit languages too much, and specific error codes are a rarity in esolangs. Any suggestions for how to fix that?

A problem with your example is that can only be reproduced with a non-standard compiler for that language. So you would have to specify to what extent non-standard language extensions are allowed and if your program relies on them... which in turn would be a major spoiler. Running main; on a conforming C compiler gives me no less than 5 errors, where several are "constraint" or "shall" violations of the language. Basically, it is very far from valid C language.

However, chasing down a specific poorly-specified program output to a certain compiler, system + options could perhaps be an interesting and very different challenge. If you write a program that purposely does something bad, resulting in crazy results, then task people to find a system and compiler that reproduces those results. Like: _Bool b=0; *(unsigned char*)&b = 42; printf("%d %d", b, b!=0); gives output 42 42. Find a system and compiler that reproduces this behavior.