What now? If your results are inconclusive or inconsistent with "what should have happened" you have an accuracy problem. Here are some things to consider:
Never, never, never say something like "this discrepancy is due to human error". You will not receive any credit for a lab in which you do this. Here's why:
Science is thinking - the most powerful tool any a scientist will ever possess is her brain. Neglecting or refusing to use your brain is the most unscientific thing that you could ever possibly do. Saying "Oh well, human error..." is equivalent to saying "It isn't obvious to me what is going on here, and I don't care enough to put any effort into thinking about it." Science would never have made any progress at all if such an attitude were acceptable.
In much the same vein, many students are accustomed to automatically attributing experimental discrepancies to "friction" or "air resistance". Friction and/or air resistance may indeed be a factor in your results (see below), but if so, you are expected to provide evidence, or at least a plausible argument, for their effect.
In other words, keep in mind that you are expected to think about your results!
You don't have to be a doofus to make a mistake, and mistakes certainly affect the accuracy of experimental results. There are some things to do to minimize mistakes:
Mistakes happen, but that doesn't mean that most mistakes can't be taken care of before they foul up the final results.
Here are some things to consider if you think you may have made a mistake:
Systematic uncertainties affect the accuracy of a result by producing measurements that are consistently too high or too low. For instance, wood expands and shrinks due to temperature and humidity. Therefore, the length of a pendulum that you measure with a meter stick (as well as the length of the wooden meter stick!) is probably either a little too long or a little too short - depending on the temperature and humidity in the room. There is little or nothing that we can do to prevent this, and in many cases these uncertainties are not significant.
Other examples of systematic error are meters that consistently read too high or too low, parallax errors caused by reading a scale from an angle, and bad connections that introduce extra resistance into a circuit.
In order to make problems possible to solve, physicists routinely make simplifying assumptions. We may have assumed that friction or air resistance was not a factor in order to calculate a theoretical value, but friction and air resistance are often factors in a real experiment.
To derive the equation for the period of a simple pendulum, you assume:
None of these assumptions are strictly true for a real pendulum, although a real pendulum can be set up to minimize most of the error introduced by these assumptions - the equation for the period of a pendulum "works" to high precision.
The point is, accuracy problems are not always (or completely) the fault of the experimental setup or the experimental technique.
If you suspect that one or more of these factors are affecting your results, you need, at minimum, to make some reasonable quantitative estimates of their effect, and provide a plausible justification for your estimate. Even better, if time and equipment permit, would be some experimental justification for your estimate. Still better, can you modify your experiment to account for, or eliminate, these effects? If time and/or resources don't permit revising your experiment, you can at least discuss some possible methods for accomplishing this task.