Authorities perform extensive investigations, verifications. Chances of any FALSE accusation of fraud or misrepresentation are nearly impossible. Much more commonly, they do miss some incidences of fraud or misrepresentation. But of course, if such evidence does crop up later, they can revoke any visa with deportation/ mandatory departure & permanent bar to entry. Canada is particularly rigorous in their requirements for evidence of fraud, misrepresentation, intent to remain, etc.