To make sure that it yields the same result each time.
error
Error
donut
to keep track of an experiment
Error
ERROR
redo
Experimental error.
The will repeat the experiment, if they can. Sometimes with a very expensive experiment, like a drug trial, there may be steps they can take to mitigate the effects of a mistake. With the Hubble Space Telescope, for example, we were able to attach a lens to correct for a mistake in the primary mirror. Other times the experiment just has to be repeated. That is often the case anyway, as experimental results are more reliable when they are verified and corroborated by an independent group. Most scientific experiments end in failure, and sometimes external factors do show up post-research.
Well, you're going to need the right things and also you need to have experience if you are going to design an experiment.
Whatever experiment scientists do, when they make a mistake they obviously need to do the whole experiment again!
A hypothesis doesn't necessarily need to be correct. After a scientist has conducted an experiment and discovered that their hypothesis is incorrect, they still have gained the knowledge and the results from their experiment---as well as the correct answer, in some cases. They can use the results from the experiment that tested the original hypothesis to form a new experiment.