In your lab books you will be asked to estimate the uncertainty of your measurements. This can often be a tedious task, but without it your results are meaningless. Consider the measurement of some quantity. There are really several different types of uncertainties. Systematic uncertainties are uncertainties that arise because of faulty alignment of equipment or improper accounting for external effects.
Systematic uncertainties are perhaps the hardest to find and adequately track down.
One needs to do a bit of thinking to try and grasp all of the possible problems that might arise. Often it is a good idea to propose simple tests that would verify that a particular systematic is not a problem. For example, in the case of the voltmeter, one could check your meter with another one, or disconnect it to insure that when the voltage was truly zero, it read zero. The more precise you are trying to be, the more difficult these test become. One can also make educated guesses of the systematic uncertainty in a measurement.