Calculators are a fantastic tool for everyone to arrive at answers especially when working with large numbers. It is easier and more reliable than using paper and pencil.
The biggest problem with calculators though is the user. Students are quick to type in numbers and then call out the answer. Few students ever stop to think about the answer given and whether it is correct or not. They never think to challenge the answer. Of course it not the calculator that makes the mistake, it is the user. What is typed in is critical to whether the answer is right or not. This is where knowing number facts come in. A student fluent with number facts, their extensions, fraction use, percentages and the like, can see immediately that the answer they have in front of them doesn’t make sense. It can’t possibly be right because answer is mathematical nonsense. There has been a mistake made somewhere and the student will have to go back over it and find the mistake that was made and recalculate the answer.
In short students who are fluent in their number facts is armed to think about whether the calculator answer is right or not.