There is a tendency of people to use 'ad hominem' as a defence, when in fact it isn't an actual ad hominem.
Example: "Your 9-11 Truther beliefs are garbage because you are an idiot." IS an ad hominem. but "You are a jerk, and your 9-11 Truther beliefs are garbage." is not an ad hominem.
The difference being that being wrong consequent to being what ever the alleged attack is is not using a logical fallacy; whereas being wrong because of the implications of the attack is a logical fallacy.
In fact, decrying 'ad hominem' when in fact there is no causal chain between being wrong and the personal attack is in a sense a logical fallacy in it's own right.
Is there a name for this? Does it fit cleanly into one of the other common established fallacies? Or is it it's own (or a subset of another) fallacy?