In the scientific process, we should not rely on the results of a single test. Instead, we should perform the test over and over. Why? If it works once, shouldn't it work the same way every time? Yes, it should, so if we repeat the experiment and get a different result, then we know that there is something about the test that we are not considering.
In studying the processes of science, you will often run into two words, which seem similar: Repetition and Replication
Sometimes it is a matter of random chance, as in the case of flipping a coin. Just because it comes up heads the first time does not mean that it will always come up heads. By repeating the experiment over and over, we can see if our result really supports our hypothesis (What is a Hypothesis?), or if it was just random chance.
Sometimes the result might be due to some variable that you have not recognized. In our example of flipping a coin, the individual's technique for flipping the coin might influence the results. To take that into consideration, we repeat the experiment over and over with different people, looking closely for any results that don't fit into the idea we are testing.
Results that don't fit are important! Figuring out why they do not fit our hypothesis can give us an opportunity to learn new things, and get a better understanding of the idea we are testing.
Once we have repeated our testing over and over, and think we understand the results, then it is time for replication. That means getting other scientists to perform the same tests, to see whether they get the same results. As with repetition, the most important things to watch for are results that don't fit our hypothesis, and for the same reason. Those different results give us a chance to discover more about our idea. The different results may be because the person replicating our tests did something different, but they also might be because that person noticed something that we missed.
What if you are wrong!
If we did miss something, it is OK, as long as we performed our tests honestly and scientifically. Science is not about proving that "I am right!" Instead, it is a process for trying to learn more about the universe and how it works. It is usually a group effort, with each scientist adding her own perspective to the idea, giving us a better understanding and often raising new questions to explore.