Accepting the four pramanas as sound means of knowledge does not, for Nyaya, imply that they are infallible. What Nyaya claims about the pramanas is that in general the four pramanas can be trusted and that as a matter of fact they are the four sources from which our knowledge is derived. Nyaya accepts the existence of error. Most discussion of error in Indian philosophy centres on the possibility and the nature and explanation of perceptual error . There is little difficult in accounting for error that arises from use of unsound forms of reasoning or from misuse of sound forms.
Perceptual error is more problematic.
The Nyaya account of perceptual error is that error consists in perceiving something as other than it is. Error is always subjective. Indian philosophers use standard examples to make their positions clear. One standard example of perceptual error is the rope-snake. Nyaya philosophers see memory is a source of knowledge, it is the perception of traces of past experience has left on the mind.