Manning's Corollary to Godwin's Law

Saw this at Mark Shea's blog. I'd never heard of "Manning's Corollary to Godwin's law" before, although I'm glad it has a name:

In any online conversation about an incident of violence perpetrated by adherents of Islamic fundamentalism, the conversation will inevitably devolve into claims that Christians commit the same type and degree of violent acts, regardless of how demonstrably false that is; further, the claim will be made that past historical violence involving Christians means that present-day Christians are morally incapable of denouncing current violence involving Muslims.