The experimenter was lying.
The experimenter was drunk.
The experimenter was insane.
The experimenter was hallucinating.
The experimenter was tricked by some specific (but unproven) ruse.
The experimenter was tricked by some unknown ruse that may be determined in the future.
The experimental protocol was flawed in some specific (but unproven) way.
The experimental protocol was flawed in some unknown way that may be determined in the future.
The equipment malfunctioned.
The photos (or videotape, etc.) were faked.
The witnesses were in cahoots with the experimenter.
The experimenter was in cahoots with the test subject.
The results were a meaningless fluke.
What struck me by this list is that just about the same reasons those objectivists who worship Ayn Rand use for rejecting Evolution, global warming, etc. They have their minds made up don't confuse it with facts.
I'm not much into psychic healing, communicating with the dead, premonitions, and other kinds of mysticism, but this list is about denial not skepticism.