"Extraordinary claims require extraordinary evidence" (or "ECREE") -- common atheist saying, based on David Hume's maxim1.
There are several problems with ECREE, the first being that it is demonstrably false. When David Hume wrote his essay, Of Miracles, from which this soundbyte is derived1, he was not aware of the rigorous method that we can use to test truth claims. This method, called Bayesian Inference, is beyond the scope of this blog post, but suffice it to say that the unlikelihood of the evidence is not the only probability you must weigh when determining whether there is evidence for a miracle. You must also weigh how likely it would be for the evidence to be as observed if the miracle did not actually occur. In some (but definitely not all) miracle claims, this can easily outweigh the unlikelihood of a miracle.
The second problem for ECREE is that its reasoning is circular. It is saying that, in order to prove a miracle, you have to demonstrate another miracle. But how will you ever prove the second miracle? Well, of course you'd need a third miracle. And to prove the third? You get the picture. It seems to me the use of this maxim, while not only an improper way of determining probabilities, is also a convenient philosophical shield against honestly considering miracle claims in the first place. It is assuming such a high price to prove a miracle that one can never be proved.
1 - Hume's actual quote is here: "[N]o testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavors to establish".