Adding ad hoc hypothesis to resist falsification
In the Popperian view of epistemology, the key ingredient of knowledge-growth is criticism. To contribute to the growth of knowledge, the creators of theories should seek out criticism of their theory. One way to do this is to seek out logical inconsistencies in it. Another way is to conduct experiments that would either falsify or corroborate the theory.
The anti-scientific stance in this view would be to resist criticism. One common way to do this (you see this often with conspiracy theorists) is to keep modifying the theory every time some evidence arises to contradict it. Say you’ve come up with some equations for the motion of all objects, and then you find that pink feathers do not follow this equation. The conventionalist (as Popper calls it, someone who wants to preserve the existing theory) will say, “no worries, just add an additional hypothesis to the theory that goes: pink feathers will move according to this other equation Y.”
As more and more counterexamples come up, the conventionalist adds more and more exceptions to their theory. At some point this becomes futile. The better approach is to consider the old theory dead, and to try to come up with a new theory that explains both the old and the new observations.