Solve the equation $2^{2x} - 5(2^x) + 4 = 0$.

Let $y = 2^x$, so $2^{2x} = y^2$ and the equation becomes $y^2 - 5y + 4 = 0$, giving $(y-1)(y-4) = 0$, so $y = 1$ or $y = 4$. Then $2^x = 1$ gives $x = 0$ and $2^x = 4$ gives $x = 2$. Stopping at $y = 1, 4$ and reporting those as the values of $x$ is the common error.