This should be codified as a named rule on the internet akin to Godwin's Law. Ad Hominim is too general.

Actually I thought that too. We should call it River's Law. (Styx's Law would presumably be something about hair bands.)

So because I never really read usernames while reading comments, my mental parser read this as River [Tam's] Law, which was apparently not immediately rejected as incorrect because she is a bit different in the mental department and would probably react... appropriately to the word "retarded".

As much as I appreciate river_styx's contribution to the development of River's Law, I really think my mental flight of fancy is superior. Or at the very least would make for a better motivational poster:

"Don't Say 'Retarded', Or She'll Kill You With Her Brain"

Point taken. Feel free to substitute a softer adjective of your choice then. ;-)

Be clear though that I was attacking the argument itself, not the poster.

Now, if Hitler had written what you wrote, he would still be just as wrong. And irredeemably evil. Therefore, you are a child murderer.