Simplify $\sqrt{50} + \sqrt{18} - \sqrt{8}$.

Express each surd in terms of $\sqrt{2}$: $\sqrt{50} = 5\sqrt{2}$, $\sqrt{18} = 3\sqrt{2}$ and $\sqrt{8} = 2\sqrt{2}$. Then $5\sqrt{2} + 3\sqrt{2} - 2\sqrt{2} = 6\sqrt{2}$. Adding all three coefficients instead of subtracting the last gives the tempting but wrong $10\sqrt{2}$.