Don't take my word for it. According to apostate atheist Hector Avalos, in "Can Science Prove that Prayer Works?" Free Inquiry 17 (1997):
Even if we saw an extraordinary healing occur (e.g., a severed leg grow back instantaneously), we would not be able to prove scientifically that it was a supernatural occurrence.
For most of my young and adolescent life, I was a faith healer in a Pentecostal tradition. I witnessed what I then thought were resurrections, spontaneous growth of short limbs, cures from cancer, and many other types of diseases.
So he's conceding that he saw the instantaneous regeneration of amputated limbs. (Notice that he uses "spontaneous" as a synonym for "instantaneously".) By his own admission, that's from firsthand observation.
He doesn't deny what he saw. "Who should I believe–me or my dying eyes!" Instead, he says that's still not scientific proof that it was a supernatural occurrence.
Now, Avalos is such a fanatical atheist that he might backpedal on his original, damaging admission. Again, though, how could he be mistaken? How could he see an amputated limb merely appear to instantaneously grow right before his eyes?
Notice that he's not talking about tricks by other faith-healers, but his own direct observation.