Certainly any thermometer can go off. Column separation due to rough handling is a common problem. Take a good magnifying lens or jeweler's loupe and check the entire column. If you can get all the liquid into the bulb you can correct this problem.
Column separation is the only problem that can occur. Anything else involves breakage of the tube. The easiest way to fix it is to cool the thermometer until all the fluid is in the ball, but it can be problematic when the thing goes down to minus 40.
An error of 8 degrees would involve a single very apparent separation, or a less apparent series of separations.