I have one family where at one census son Joseph is 18 and working, and the next census has him aged 8 and a scholar. William goes from 10 to 30 in the same period.
It turns out that the second Joseph was the illegitimate son of one of the daughters of the household. His uncle Joseph had married and was living a few miles away.
The first William was the illegitimate son of another daughter. He had moved out by the second census, and his uncle William had returned from working elsewhere.
It only became clear when I looked at the parish registers.