What are the limitations if you conducted experiment one time?

one limitation is a very small data set (it's hard to tell if the result is going to be different next time). Which brings up the subject of variability - are the initial conditions always the same (will my beaker be a little dirty or contaminated next time, will my lab partner click the stopwatch at exactly the same time, will I be distracted at the end of the experiment and miss an important detail...).

A good experiment is repeatable, a bad one gets different results each time.