Would this indicate new physics if proven?
Maybe, but it would really shake up the scientific method...
It is an interesting attack, and IANAL, but I'd be curious about the legal ramifications. If I slip a carbon (ah... the way-back machine) in a stack of papers and ask someone to sign the top one without thus informing them, I think my stealth probably invalidates the additional document(s).
You could argue that there's a noticeable difference between pen and carbon -- making the copy hard to enforce -- but I'd argue the digital version is even easier: at least in the PS example, both "copies" of the document need to be present to preserve the hash.
In normal (pen/paper) signature situations, I get a copy of what I signed. The same ought to apply to digital sigs, resulting in a simple legal challenge to the validity of the document.
It is easier to write an incorrect program than understand a correct one.