What would happen if you wear far sighted glasses but you're nearsighted?

More nearsighted: If you were nearsighted (myopic), but were wearing glasses which were intended to correct farsightedness (hyperopia), your vision while you wore those glasses would be even more myopic, and distant targets would appear even blurrier than with no glasses at all. This should not produce any lasting ill-effects unless you were under 15 years of age, but i still wouldn't recommend it!