The canceling hypotheses fallacy occurs when someone defends their hypothesis by proposing a second hypothesis to explain the lack of evidence in support of the first hypothesis. In effect, the second hypothesis cancels or undermines the prediction made by the first hypothesis.
Flat-Earthers like to demonstrate what they regard as ‘evidence’ of a flat Earth. However, when it is demonstrated that the evidence is wrong, they would invent a second hypothesis to defend the original hypothesis against the evidence of the contrary. They do it without attempting to bring sufficient proof of the second hypothesis.