[kalkin]

This isn't correct. What you may be remembering is that some (not all) NP complete problems have limits on how accurately they can be approximated (unless P = NP). But approximation algorithms for NP complete problems form a whole subfield of CS.

[moyix]

The theorem that proves this is the PCP Theorem, in case anyone wants to read more about it: https://en.wikipedia.org/wiki/PCP_theorem#PCP_and_hardness_o...