It looks that the default Hash implementation depends on whether the system is 32-bit or 64-bit [1]. It's also not clearly documented what the default implementation of Hash really is.

On a 32-bit system, yes, someone could probably do it.

[1] https://mathematica.stackexchange.com/a/124340

Probably. There are only 2^64 possible hash values (Mathematica's Hash function, with no method specified, produces a 32- or 64-bit output depending on platform), and there are many more strings than that which could reasonably be called a "paragraph". It's pretty reasonable to hope that there's a self-describing paragraph out there somewhere.

Hah, "could in theory" vs "could in practice" -- depending on the hash function, it could take hundreds or thousands of processor-years. Maybe Google could do it.

http://m.wolframalpha.com/input/?i=2%5E64+things+%2F+%281+bi...

Actually for any given paragraph, probabilty of hash(paragraph+hash) = hash is pretty high.

You mean that there exists at least one hash such that hashFunc(paragraph+hash) = hash? Yup, that probability converges to 1-exp(-1) = 63%. So it's no guarantee, but if you have some wiggle room in the paragraph (change the wording slightly etc) you can probably make it work.

Given that they control the hash function, it would be trivial: hard-code a special exception value for exactly that purpose.