the first one I poorly worded. I referred to the trick of e.g. arranging things such that the logical start of an array is before the region you actually allocated. It is true that as I wrote that it didn't make sense.
The rest of them I found in the spec I linked; yes, most of them are implementation-defined.
I mean, I think you and I agree about the world we would rather live in, where C compilers check memory constraints usefully.