Double counting is the error that leads to an overestimation of national income. It occurs when the value of a good is counted more than once as it passes through different stages of production.
For example, if a farmer sells wheat to a flour mill, the mill sells flour to a bakery, and the bakery sells bread to consumers, the value of the wheat is embedded in the flour price, which is in turn embedded in the bread price. If you add the sales value at every stage without subtracting intermediate inputs, you count the wheat's value three times, the milling value twice, and only the baking value once. The resulting total far exceeds the actual value of final goods produced, inflating the national income figure.
To avoid this, national income accountants use either the value-added method (counting only the value added at each stage) or count only the value of final goods and services.
Wrong timing of computation may shift income between periods but does not systematically inflate the total. Changes in prices within the year can distort comparisons between years but are handled by using constant prices. Incomplete statistical data would, if anything, lead to an underestimate because unrecorded economic activity is omitted from the count.