If you did not follow the procedure as you should have.
Explanation:
You may have to repeat an experiment for several reasons. It can be because you need to be certain of the results so that you have to perform it several times. Or you need more precise results so that you can fine-tune the variables and repeat the experiment again.
The most common scenario where an experiment would be replicated would be in a situation where you did not follow the procedure that you have outlined exactly like you should have so that the results you got are not 100% reliable. In such a case, the experiment would need to be replicated for the correct results.
