A lie can travel half way around the world while the truth is putting on its shoes. And obviously there are an infinite number of ways to save a pet theory from being proven wrong. Ad hoc changes to definition and ad hoc hypothesis can be introduced at an incredible rate.
As phrased by Karl Popper in The logic of scientific discovery:
“… it is always possible to find some way of evading falsification, for example by introducing ad hoc an auxiliary hypothesis, or by changing ad hoc a definition. It is even possible without logical inconsistency to adopt the position of simply refusing to acknowledge any falsifying experience whatsoever. Admittedly, scientists do not usually proceed in this way, but logically such procedure is possible”
– Karl Popper
Hence the attempts to evade falsification are ruled out by the empirical method, the hypotetico – deductive model, the modern scientific method:
“the empirical method shall be characterized as a method that excludes precisely those ways of evading falsification which … are logically possible. According to my proposal, what characterizes the empirical method is its manner of exposing to falsification, in every conceivable way, the system to be tested. Its aim is not to save the lives of untenable systems but … exposing them all to the fiercest struggle for survival.»
Regarding the position of simply refusing to acknowledge any falsifying experience whatsoever – the modern scientific method gives no clue about how to handle that.
Monthy Python however – gave it a pretty good shot: