disagree, some small size NP-hard problems can be solved in practise. or, given enough effort / luck / redefining the problem, sometimes extra structure can be

disagree, some small size NP-hard problems can be solved in practise. or, given enough effort / luck / redefining the problem, sometimes extra structure can be found for the particular problem instances that you want to solve, meaning that they actually belong to an easier problem class that can be solved in practise -- e.g. you've got extra information or constraints that you're not actually using, so you don't need to solve the NP-hard problem in general, just solve the specific problem you've got. i'll willing to change my position if someone can argue it is similar for undecidable problems.