Eric,
I realize its not a symbol. It was the fact that it crashed the system that kind of had me worried. As I said I was just playing around when I came across it. I was planning to use ##symbol-hash in an application, but if its got a chance of causing a segfault then I will avoid it (and all the others that start with ##).
I'm sure there is no problem using them, given that you properly test the type of the argument yourself before calling them. For example:
(if (symbol? v) (##symbol-hash v) ...)
Dominique